fix(desktop): queue a single pending send during streaming

[luosc]

Block desktop shortcut sends from starting a second generation while the current turn is still streaming. Queue one pending composer input instead, show a three-line preview, and allow canceling the queued item back into the editable composer with attachments restored.

Move the guard into the controller/view-model send path so keyboard shortcuts cannot bypass the stop-state protection, and automatically dispatch the queued input when the active generation finishes or is stopped.

Closes #458

Files changed:

• lib/core/models/chat_input_data.dart: add single-queue submission result and queued input model
• lib/features/home/controllers/home_page_controller.dart: return send submission results and restore queued input back into the composer on cancel
• lib/features/home/controllers/home_view_model.dart: add single pending-send queue, block concurrent sends, and auto-drain after streaming stops
• lib/features/home/pages/home_page.dart: pass queued input state and preview text into the shared chat input section
• lib/features/home/widgets/chat_input_bar.dart: make send async-result aware, lock the composer while queued, and render queued preview/cancel UI
• lib/features/home/widgets/chat_input_section.dart: thread queued-input state and preview props into ChatInputBar
• lib/l10n/app_en.arb: add queued pending/cancel strings
• lib/l10n/app_localizations.dart: regenerate localization accessors for queued-input strings
• lib/l10n/app_localizations_en.dart: regenerate English queued-input strings
• lib/l10n/app_localizations_zh.dart: regenerate Chinese queued-input strings
• lib/l10n/app_zh.arb: add queued pending/cancel strings
• lib/l10n/app_zh_Hans.arb: add queued pending/cancel strings
• lib/l10n/app_zh_Hant.arb: add queued pending/cancel strings
• test/features/home/widgets/chat_input_bar_queue_test.dart: cover queued send clearing, rejected send retention, and queued preview/cancel UI