feat: add supported shortcut parity

[salmonumbrella]

Summary

Adds the supported Superhuman-style shortcut matrix to the registry-owned command engine while preserving MailFlow behavior, user overrides, native styling, and the dependency-free architecture.

This is the fourth stacked command-system PR. It includes prerequisite commits from #335, #337, and #339; review the last commit only (fc2ce58).

Closes #340.

Changes

• adds one context-aware desktop dispatcher for sequences, platform modifiers, rank-based conflicts, IME safety, and modal/typing isolation
• adds contextual Enter/O, J/K and N/P navigation, Space scrolling, selection shortcuts, Escape back, G sequences, Undo, Unsubscribe, and supported mail/GTD bindings
• routes navigation and selection through the shared store-backed targets used by Command-K and mail executors, including thread-cache fallback and preserved mark-read behavior
• preserves legacy user overrides at read time and displays primary, secondary, platform, and custom sources plus every collision in Settings and shortcut Help
• opens Move/Snooze continuations immediately, keeps Compose send editor-owned, and isolates Command-K, Compose, Settings, Help, and Contacts from background shortcut execution
• adds no backend route, migration, runtime dependency, or cmdk dependency

Testing

• cd frontend && npm test — 1,643 passed
• cd frontend && npm run lint — zero warnings
• cd frontend && npm run build — passed (existing chunk-size advisory only)
• candidate frontend/backend Docker images built successfully
• inert shortcut scan and nine deferred-capability issue audit passed
• git diff --check 6a604730..HEAD — clean
• verified exactly one PR-specific commit over feat: unify shared mail actions #339

Interactive macOS/Windows matrices remain to be performed; automated coverage exercises platform modifiers, contextual precedence, sequences, typing/modal guards, and IME composition.

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