perf(desktop): memoize composer and timeline to fix typing lag

[wesbillman]

Summary

Fix typing lag/stutter in the message composer caused by unnecessary React re-renders cascading from AppShell through the entire component tree on every keystroke.

Problem

When typing in the MessageComposer, every keystroke triggered re-renders of:

• ChannelPane (not memoized)
• MessageTimeline (not memoized — potentially hundreds of message rows)
• MessageComposerToolbar (not memoized)
• All autocomplete components

Additionally, inline arrow functions in AppShell (onSend, onCancelReply, onReply, onToggleReaction, onTargetReached) created new references every render, defeating any potential memoization.

Solution

Component memoization (React.memo)

• ChannelPane — highest-impact change, prevents timeline re-renders on keystrokes
• MessageTimeline — belt-and-suspenders with ChannelPane memo
• MessageComposerToolbar — prevents toolbar re-renders during typing
• ChannelAutocomplete, MentionAutocomplete, ComposerEmojiPicker — defensive memoization

Callback stabilization

• New useChannelPaneHandlers hook — extracts handleSend, handleReply, handleCancelReply, handleToggleReaction with stable references using useCallback + ref pattern
• TanStack Query mutation refs — .mutateAsync stashed in refs since mutation objects are new references on every state transition (idle → pending → success)
• contentRef pattern in MessageComposer — callbacks read from ref instead of closing over content state, removing it from dependency arrays
• Inline handlers extracted — handleCaptureSelection, handlePaperclipClick, handleTargetReached, effectiveToggleReaction

Minor optimizations

• lineHeight cached in ref (avoids getComputedStyle on every keystroke)
• sendDisabled memoized via useMemo

Files Changed (9 files)

• desktop/src/app/AppShell.tsx — stable handler refs, extracted callbacks
• desktop/src/app/ChannelPane.tsx — React.memo
• desktop/src/app/useChannelPaneHandlers.ts — NEW stable callback hook
• desktop/src/features/messages/ui/ChannelAutocomplete.tsx — React.memo
• desktop/src/features/messages/ui/ComposerEmojiPicker.tsx — React.memo
• desktop/src/features/messages/ui/MentionAutocomplete.tsx — React.memo
• desktop/src/features/messages/ui/MessageComposer.tsx — ref-based callbacks, cached lineHeight
• desktop/src/features/messages/ui/MessageComposerToolbar.tsx — React.memo
• desktop/src/features/messages/ui/MessageTimeline.tsx — React.memo

Future Work (low priority)

• Memoize MessageComposer itself + stabilize computed props in ChannelPane
• Memoize TypingIndicatorRow
• Handle lineHeightRef invalidation on theme/font-size changes

Testing

• tsc --noEmit ✅
• biome check ✅ (only pre-existing warnings)
• Full CI suite passed on push (rust fmt, clippy, tests, desktop build, Tauri check)