[HOLD] Decompose ReportActionComposer

[adhorodyski]

@rlinoz

Explanation of Change

Decomposes the 808-LOC ReportActionCompose monolith into a compound component architecture with optimized render performance.

Compound components:
The orchestrator is now 55 LOC — pure composition with Composer.Box, Composer.DropZone, Composer.LocalTime, Composer.Footer, Composer.ActionMenu, Composer.Input, Composer.EmojiPicker, and Composer.SendButton. Each component owns its behavior and self-subscribes to Onyx data per CLEAN-REACT-PATTERNS-2.

6 contexts organized by change frequency:

• Text (hot) — composer value, changes every keystroke
• State (warm) — isFocused, isMenuVisible, isFullComposerAvailable
• Actions (frozen) — stable setters and ref-closing callbacks, never triggers re-renders
• SendState (warm) — isSendDisabled, exceededMaxLength, isBlockedFromConcierge — all primitives, no function refs
• SendActions (infrequent) — handleSendMessage, onValueChange
• Meta (frozen) — refs only, never triggers re-renders

Hook split for render isolation:

• Split useAttachmentUploadValidation into useAttachmentPicker (lightweight, no Onyx subs) and useReceiptDrop (heavy, ~10 Onyx subs). Each consumer calls its hook directly instead of reading from context.
• Relocated useComposerSubmit from provider to ComposerInputWrapper (its sole consumer), removing ~12 Onyx subscriptions from the provider render path.
• Extracted useComposerFocus — owns focus state internally.

Activity API for hidden subtrees:

• Wrapped PopoverMenu in AttachmentPickerWithMenuItems with <Activity mode> — React skips reconciliation of the entire menu subtree when closed.
• Wrapped EmojiPicker popover with <Activity mode> — same optimization for the global emoji picker.

Performance (React Profiler, averaged across 3-4 runs):

Scenario	Main	Branch	Delta
Open menu + close (duration)	301ms	208ms	-31%
Open menu + close (commits)	34	26	-24%
Type + send message (duration)	444ms	388ms	-13%

Fixed Issues

$ #87097
PROPOSAL:

Tests

1. Open any chat and verify the composer renders correctly (input, + button, send button, emoji picker)
2. Type a message and send — verify it posts with no errors
3. Open the action menu (+) — verify all options appear, close with Escape
4. Attach a file via the action menu — verify preview modal appears and file sends
5. Open the emoji picker — verify selecting an emoji inserts it
6. Drag and drop a file onto the composer — verify it's accepted
7. Navigate between chat rooms — verify composer state resets per room
8. Test offline mode — verify composer works and queues messages
9. Verify no JS console errors throughout

[cristipaval]

FYI: I've just added #87097 to the Fixed issues section in the PR description