fix(mobile): discover, browse, and join open channels

mobile/lib/features/channels/channel.dart

@@ -79,6 +79,8 @@ class Channel {
   bool get isPrivate => visibility == 'private';
   bool get isArchived => archivedAt != null;
 
+  bool get canJoin => visibility == 'open' && !isArchived && !isMember && !isDm;
+
   String displayLabel({String? currentPubkey}) {
     if (!isDm || participants.isEmpty) {
       return name;

mobile/lib/features/channels/channels_page.dart

@@ -54,7 +54,7 @@ part 'channels_page/community.dart';
 part 'channels_page/quick_actions.dart';
 part 'channels_page/quick_actions_launcher.dart';
 
-enum _QuickAction { createChannel, newDm }
+enum _QuickAction { createChannel, newDm, browseChannels }
 
 const double _kChannelSectionInset = Grid.gutter;
 const double _kChannelLeadingWidth = 22.0;

mobile/lib/features/channels/channels_page/body.dart

@@ -196,7 +196,9 @@ class _SliverChannelsList extends HookConsumerWidget {
       sliver: SliverList.list(
         children: [
           if (visibleChannels.isEmpty)
-            const _EmptyState()
+            _EmptyState(
+              channels: channels.where((channel) => channel.canJoin).toList(),
+            )
           else ...[
             // Starred channels (exclusive — pinned above all sections).
             if (starredStreamChannels.isNotEmpty)

mobile/lib/features/channels/channels_page/quick_actions.dart

@@ -7,7 +7,7 @@ const _kMorphCloseCurve = Cubic(0.22, 1, 0.36, 1);
 const double _kMorphOpenBounce = 0.14;
 const double _kMorphCloseBounce = 0.06;
 const double _kMorphClosedSize = 56;
-const double _kMorphOpenHeight = 160;
+const double _kMorphOpenHeight = 216;
 const double _kMorphOpenRadius = 20;
 const double _kMorphSlide = 40;
 const double _kMorphScale = 0.97;
@@ -274,6 +274,13 @@ class _QuickActionsMenu extends StatelessWidget {
             key: const Key('quick-action-new-dm-card'),
             onTap: () => onSelected(_QuickAction.newDm),
           ),
+          const SizedBox(height: Grid.xxs),
+          _QuickActionItem(
+            icon: LucideIcons.search,
+            title: 'Browse channels',
+            key: const Key('quick-action-browse-channels-card'),
+            onTap: () => onSelected(_QuickAction.browseChannels),
+          ),
         ],
       ),
     );

mobile/lib/features/channels/channels_page/quick_actions_launcher.dart

@@ -107,6 +107,8 @@ class ChannelQuickActionsLauncher extends HookConsumerWidget {
           if (opened != null && context.mounted) {
             await openChannel(opened);
           }
+        case _QuickAction.browseChannels:
+          await _showBrowseChannelsSheet(context);
       }
     }
 

mobile/lib/features/channels/channels_page/sections.dart

@@ -360,29 +360,48 @@ class _ChannelSection extends StatelessWidget {
 }
 
 class _EmptyState extends StatelessWidget {
-  const _EmptyState();
+  final List<Channel> channels;
+
+  const _EmptyState({required this.channels});
 
   @override
   Widget build(BuildContext context) {
-    return SizedBox(
-      height: MediaQuery.sizeOf(context).height * 0.55,
+    return ConstrainedBox(
+      constraints: BoxConstraints(
+        minHeight: MediaQuery.sizeOf(context).height * 0.55,
+      ),
       child: Center(
-        child: Column(
-          mainAxisSize: MainAxisSize.min,
-          children: [
-            Icon(
-              LucideIcons.messagesSquare,
-              size: Grid.xl,
-              color: context.colors.onSurfaceVariant,
-            ),
-            const SizedBox(height: Grid.xs),
-            Text(
-              'No conversations yet',
-              style: context.textTheme.bodyLarge?.copyWith(
+        child: Padding(
+          padding: const EdgeInsets.symmetric(horizontal: Grid.gutter),
+          child: Column(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              Icon(
+                LucideIcons.messagesSquare,
+                size: Grid.xl,
                 color: context.colors.onSurfaceVariant,
               ),
-            ),
-          ],
+              const SizedBox(height: Grid.xs),
+              Text(
+                'No conversations yet',
+                style: context.textTheme.bodyLarge?.copyWith(
+                  color: context.colors.onSurfaceVariant,
+                ),
+              ),
+              if (channels.isNotEmpty) ...[
+                const SizedBox(height: Grid.xs),
+                Text(
+                  'Join an open channel to start a conversation.',
+                  textAlign: TextAlign.center,
+                  style: context.textTheme.bodyMedium?.copyWith(
+                    color: context.colors.onSurfaceVariant,
+                  ),
+                ),
+                const SizedBox(height: Grid.xs),
+                _JoinableChannelList(channels: channels),
+              ],
+            ],
+          ),
         ),
       ),
     );

mobile/lib/features/channels/channels_page/sheets.dart

@@ -2,6 +2,186 @@ part of '../channels_page.dart';
 
 const int _defaultCreateChannelTtlSeconds = 7 * 24 * 60 * 60;
 
+Future<void> _showBrowseChannelsSheet(BuildContext context) =>
+    showModalBottomSheet<void>(
+      context: context,
+      constraints: _quickActionSheetConstraints(context),
+      isScrollControlled: true,
+      showDragHandle: true,
+      builder: (_) => const _BrowseChannelsSheet(),
+    );
+
+class _BrowseChannelsSheet extends StatelessWidget {
+  const _BrowseChannelsSheet();
+
+  @override
+  Widget build(BuildContext context) {
+    return Consumer(
+      builder: (context, ref, _) {
+        final channelsAsync = ref.watch(channelsProvider);
+        final channels = channelsAsync.asData?.value
+            .where((channel) => channel.canJoin)
+            .toList();
+
+        return SafeArea(
+          top: false,
+          child: Padding(
+            padding: const EdgeInsets.fromLTRB(
+              Grid.gutter,
+              0,
+              Grid.gutter,
+              Grid.xs,
+            ),
+            child: ListView(
+              shrinkWrap: true,
+              children: [
+                Text(
+                  'Browse channels',
+                  style: context.textTheme.titleLarge?.copyWith(
+                    fontWeight: FontWeight.w600,
+                    letterSpacing: -0.3,
+                  ),
+                ),
+                const SizedBox(height: Grid.half),
+                Text(
+                  'Join an open channel to add it to your conversations.',
+                  style: context.textTheme.bodyMedium?.copyWith(
+                    color: context.colors.onSurfaceVariant,
+                  ),
+                ),
+                const SizedBox(height: Grid.xs),
+                if (channelsAsync.isLoading && channels == null)
+                  const Padding(
+                    padding: EdgeInsets.all(Grid.sm),
+                    child: Center(child: BuzzLoadingIndicator()),
+                  )
+                else if (channelsAsync.hasError && channels == null)
+                  Padding(
+                    padding: const EdgeInsets.symmetric(vertical: Grid.sm),
+                    child: Text(
+                      'Could not load open channels.',
+                      textAlign: TextAlign.center,
+                      style: context.textTheme.bodyMedium?.copyWith(
+                        color: context.colors.onSurfaceVariant,
+                      ),
+                    ),
+                  )
+                else if (channels == null || channels.isEmpty)
+                  Padding(
+                    padding: const EdgeInsets.symmetric(vertical: Grid.sm),
+                    child: Text(
+                      'No open channels available to join.',
+                      textAlign: TextAlign.center,
+                      style: context.textTheme.bodyMedium?.copyWith(
+                        color: context.colors.onSurfaceVariant,
+                      ),
+                    ),
+                  )
+                else
+                  _JoinableChannelList(
+                    channels: channels,
+                    closeAfterJoin: true,
+                  ),
+              ],
+            ),
+          ),
+        );
+      },
+    );
+  }
+}
+
+class _JoinableChannelList extends StatelessWidget {
+  final List<Channel> channels;
+  final bool closeAfterJoin;
+
+  const _JoinableChannelList({
+    required this.channels,
+    this.closeAfterJoin = false,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final sortedChannels = List<Channel>.of(channels)
+      ..sort((left, right) => left.name.compareTo(right.name));
+    return Column(
+      mainAxisSize: MainAxisSize.min,
+      children: [
+        for (final channel in sortedChannels)
+          _JoinableChannelTile(
+            channel: channel,
+            closeAfterJoin: closeAfterJoin,
+          ),
+      ],
+    );
+  }
+}
+
+class _JoinableChannelTile extends HookConsumerWidget {
+  final Channel channel;
+  final bool closeAfterJoin;
+
+  const _JoinableChannelTile({
+    required this.channel,
+    required this.closeAfterJoin,
+  });
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final isJoining = useState(false);
+    final actionError = useState<String?>(null);
+
+    Future<void> join() async {
+      if (isJoining.value) return;
+      isJoining.value = true;
+      actionError.value = null;
+      try {
+        await ref.read(channelActionsProvider).joinChannel(channel.id);
+        if (closeAfterJoin && context.mounted) Navigator.of(context).pop();
+      } catch (error) {
+        actionError.value = error.toString();
+      } finally {
+        isJoining.value = false;
+      }
+    }
+
+    return Column(
+      mainAxisSize: MainAxisSize.min,
+      children: [
+        ListTile(
+          key: Key('browse-channel-${channel.id}'),
+          contentPadding: EdgeInsets.zero,
+          leading: Icon(channelIcon(channel)),
+          title: Text(channel.name),
+          subtitle: channel.description.trim().isEmpty
+              ? null
+              : Text(
+                  channel.description,
+                  maxLines: 2,
+                  overflow: TextOverflow.ellipsis,
+                ),
+          trailing: FilledButton.tonal(
+            key: Key('browse-channel-join-${channel.id}'),
+            onPressed: isJoining.value ? null : () => unawaited(join()),
+            child: Text(isJoining.value ? 'Joining\u2026' : 'Join'),
+          ),
+        ),
+        if (actionError.value case final error?)
+          Align(
+            alignment: Alignment.centerLeft,
+            child: Text(
+              error,
+              key: Key('browse-channel-error-${channel.id}'),
+              style: context.textTheme.bodySmall?.copyWith(
+                color: context.colors.error,
+              ),
+            ),
+          ),
+      ],
+    );
+  }
+}
+
 class _CreateChannelMenuOption<T> {
   final Key? key;
   final String label;