feat: add desktop timeline revert controls

[anis-dr]

Summary

• add desktop session revert controls end-to-end: users can now revert directly from a turn and restore from a revert banner in the prompt dock
• create a new revert icon button inside the user message box (next to copy), including hover behavior and undo icon support
• wire session revert/unrevert handlers through session.tsx, message-timeline.tsx, and session-prompt-dock.tsx
• add session.revert.* and ui.sessionTurn.revert translations across app and UI locale files

Linked Issues

• closes Desktop/Web: Missing Revert and Fork actions for messages #9661
• related feat: add undo/fork buttons on user messages and show agent description in prompt placeholder #13265

Validation

• bun run typecheck (packages/ui)
• bun run typecheck (packages/app)
• bun test src/i18n/parity.test.ts (packages/app)

[github-actions]

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