feat: unify shared mail actions

[salmonumbrella]

Summary

Consolidates supported mail mutations behind the registry-owned command controller so Command-K, shortcuts, list and pane controls, context menus, bulk actions, hover actions, and mobile swipes share target resolution and execution behavior.

This is the third stacked command-system PR. It includes prerequisite commits from #335 and #337; review the last commit only (6a604730).

Closes #338.

Changes

• adds dependency-injected executors for archive, move, snooze, read/unread, star, trash, spam/ham, reply/forward, and GTD classification
• preserves account-scoped frozen targets, thread expansion, optimistic rollback, read/removal guards, the 4.5-second undo window, unload flushing, and exact partial-result counts
• routes the message list, message pane, context menu, bulk/hover/swipe surfaces, and compatibility shortcut events through the shared controller
• adds typed Move/Snooze continuations, native palette icons, localized aliases/outcomes across all seven locales, and regression coverage for IME-safe palette behavior retained from feat: add contextual command palette #337
• adds no backend route, migration, runtime dependency, or cmdk dependency

Testing

• cd frontend && npm test — 1,648 passed
• cd frontend && npm run lint — zero warnings
• cd frontend && npm run build — passed (existing chunk-size advisory only)
• git diff --check 20589608..HEAD — clean
• verified exactly one PR-specific commit over feat: add contextual command palette #337

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