feat: undo inbox triage actions

[salmonumbrella]

Summary

Add a configurable Cmd+Z/Ctrl+Z Inbox triage undo shortcut while preserving native text undo in composers and editable controls.

Changes

• Route the standard undo shortcut through the existing guarded shortcut dispatcher and limit it to the active Inbox message list.
• Give keyboard, context-menu, swipe, and bulk triage actions one atomic 4.5-second undo window with newest-action-first ordering.
• Share the delayed archive path across keyboard and pointer actions while preserving next-message selection and optimistic counters.
• Make newly confirmed GTD classifications reversible without removing pre-existing labels or changing COPY semantics.
• Add regression coverage for editable-target routing, rapid actions, exact expiry boundaries, non-UIDPLUS classification, and failure reconciliation.

Testing

• Frontend: npm test — 1,579 tests passed.
• Frontend: npm run lint — passed with zero warnings.
• Frontend: npm run build — passed (existing chunk-size advisory only).
• Backend: npx vitest run — 990 tests passed.
• Backend: npm run lint — passed with zero warnings.

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