feat: optimistic message rendering with reconcile

[ImpulseB23]

Closes #88. Stacked on #93.

Adds an optimistic-send path so locally-authored messages render the moment the user hits Enter, without waiting for the platform echo round-trip.

Frontend (chatStore)

• ChatMessage gains optional status (pending|failed), local_id, error_message
• New API: insertPending, confirmPendingId, failPending, retryPending, plus messageRevision signal for in-place mutation
• Reconcile incoming echo against pending tail by id (after confirm) or fingerprint (platform + username + text + 30s window)
• Pending arrival_seq sourced from MAX_SAFE_INTEGER - 1M base so they sort to tail in any future cross-platform sort layer
• buildOptimisticMessage helper builds the pending entry from {platform, login, text}

Frontend (UI)

• MessageInput now takes the user login, builds optimistic, inserts, then sends; on success calls confirmPendingId, on failure calls failPending
• ChatFeed subscribes to messageRevision and clears its prepared-layout cache on bump; pending entries render at 0.55 opacity, failed entries get a red left-border + click-to-retry
• twitchAuth.sendMessage now returns Promise<{ message_id }>

Backend

• twitch_send_message returns Result<SendChatOk, SendCommandError> with the Helix message_id so the frontend can swap the local id for the authoritative one before the echo arrives

Tests

• 11 new chatStore tests cover insertPending, reconcile-by-id, reconcile-by-fingerprint, fingerprint window expiry, cross-platform isolation, failPending, retryPending state machine, scan-tail bound, multi-pending ordering
• All 73 vitest tests + 187 cargo tests pass; clippy/fmt/tsc/eslint/prettier clean

[Copilot]

Pull request overview

Adds an optimistic-send path so locally-authored Twitch messages render immediately on Enter, then reconcile in-place with the authoritative platform echo (using Helix message_id when available, otherwise a bounded fingerprint scan).

Changes:

• Extend chatStore with pending/failed message state, reconciliation, and a mutation signal for in-place updates.
• Update UI to insert pending messages on submit, dim pending entries, and allow click-to-retry for failed sends.
• Return Helix message_id from the Rust twitch_send_message command and surface it through the frontend sendMessage API.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
apps/desktop/src/stores/chatStore.ts	Adds optimistic message model fields, pending insert/confirm/fail/retry APIs, reconcile logic, and messageRevision.
apps/desktop/src/stores/chatStore.test.ts	Adds unit tests covering optimistic insert + reconciliation behaviors and state transitions.
apps/desktop/src/lib/twitchAuth.ts	Changes sendMessage to return { message_id } on success.
apps/desktop/src/components/MessageInput.tsx	Inserts optimistic pending message before sending; confirms/fails pending based on command result; now requires login prop.
apps/desktop/src/components/ChatFeed.tsx	Invalidates prepared-layout cache on message mutations; renders pending/failed styling and supports click-to-retry.
apps/desktop/src/App.tsx	Wires authenticated login into MessageInput.
apps/desktop/src-tauri/src/sidecar_commands.rs	Updates twitch_send_message to return a success payload containing message_id.
apps/desktop/src-tauri/src/host.rs	Documents SendChatResult.message_id as frontend-consumed for optimistic reconciliation.

Comments suppressed due to low confidence (1)

apps/desktop/src/stores/chatStore.ts:222

• The createChatStore doc comment says there is a “single viewport signal”, but this change introduces an additional messageRevision signal. Update the comment to reflect the new reactive surface (or consolidate to one signal if that’s the intent).

/**
 * Creates a chat store backed by a plain pre-allocated ring buffer. Writes
 * happen synchronously; the single viewport signal is batched into one
 * `requestAnimationFrame` tick so multiple batches arriving within the same
 * frame coalesce into exactly one reactive update.

[codecov]

Codecov Report

❌ Patch coverage is 86.79245% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
apps/desktop/src/stores/chatStore.ts	86.59%	6 Missing and 7 partials ⚠️
apps/desktop/src-tauri/src/sidecar_commands.rs	88.88%	1 Missing ⚠️

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