feat: add contextual command palette

[salmonumbrella]

Summary

Adds the authenticated desktop Command-K experience on the shared command engine, with contextual application commands, accessible keyboard operation, and a MailFlow-native visual shell.

Review the last commit only; #335 is the prerequisite.

Closes #336.

Changes

• share checkbox selection through Zustand and build immutable live command contexts
• register Compose, Search, Contacts, inbox/account/folder/GTD, theme, and applicable Settings commands
• compose one registry/controller runtime with continuation and terminal-outcome handling
• rebuild the palette with localized fuzzy results, alias explanations, target disclosure, semantic icons, and right-aligned shortcut hints
• preserve the exact 5.5-row viewport while scrolling the active keyboard option into view
• trap and restore focus, expose complete dialog/listbox/active-descendant semantics, and ignore navigation/execution during IME composition
• preserve rich-editor Insert Link on the first Cmd/Ctrl+K and open the palette on the second press
• localize the complete command and palette surface across all seven supported locales
• add no dependency; filtering and ranking remain registry-owned

Testing

• cd frontend && npm test (1,634 passed)
• cd frontend && npm run lint (zero warnings)
• cd frontend && npm run build (passed; existing chunk-size advisory only)
• authenticated browser acceptance for the exact 286px / 52px = 5.5-row viewport, keyboard scrolling, alias disclosure, focus trap/restoration, ARIA state, and dark/light theme integration
• git diff --check f226af6..HEAD
• verified exactly one PR-specific commit and no dependency manifests, plans, credentials, local paths, or PII

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