feat(desktop): add focused-message and unread navigation shortcuts

[JohnRichardEnders]

Summary

Add keyboard-driven conversation handling to the standard desktop UI while preserving existing composer and terminal bindings.

• F6 moves from the composer into the message timeline without modifying the draft; unmodified up/down move between focused messages.
• On a focused message: T / right arrow replies in a thread, R opens the existing reaction picker, E edits when permitted, U marks unread, and L copies the message link. Left arrow returns from a thread to the source conversation; Escape returns to the composer.
• Add previous/next unread conversation navigation: Option+Shift+up/down on macOS, Ctrl+Alt+Shift+up/down on Windows/Linux. Uses rendered sidebar order, includes rendered DMs, skips read/muted entries, and does not wrap.
• Add Cmd/Ctrl+Shift+T for Inbox's Threads filter and Cmd/Ctrl+Shift+J for the current conversation's unread position.
• Register the bindings in shortcut help and correct strikethrough help to the existing Cmd/Ctrl+Shift+S binding.

Single-letter actions require direct message-row focus, not a nested control or editable field. IME, dialogs, modifier conflicts, existing up-arrow-to-edit, Shift+Escape, and terminal bindings retain precedence. Focus styling and scroll margins keep selected messages clear of the fixed header/composer; marking unread returns focus to the stable composer before the timeline rebuilds.

Duplicate check

Closest work: #3377 / #3078 already covers ordinary previous/next channel shortcuts and explicitly defers unread navigation. This PR does not implement or override those ordinary shortcuts. #3459 is an alternate developer-mode UI, rather than these standard-UI bindings. #6549 concerns customizable bindings, which remain outside this change. Searched issues and PRs for shortcuts, keyboard navigation, focused-message actions, and unread/thread shortcuts; no equivalent standard-UI contribution found.

Testing

• TypeScript typecheck, Biome checks on all 16 changed files, and file-size checks pass.
• 17 focused unit cases pass, covering modifier/platform precedence, focused-action availability, IME/overlay guards, and unread ordering/boundaries.
• Two retained browser regressions pass: focused messages remain visible beneath timeline chrome without altering drafts; edit/thread/mark-unread focus transitions preserve keyboard operation.
• Local Chromium smoke exercised reaction picking, copy link, unread conversation navigation, unread-position jump, Threads navigation, and shortcut help. Visually checked focus styling and help layout.

Browser verification uses the existing mock Tauri/relay bridge. Windows/Linux chord matching is unit-tested; native Windows/Linux runtime interaction was not exercised.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

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