feat: add keyboard shortcuts help overlay

[tangmingxiu11-commits]

Summary

• add a typed keyboard shortcut registry used by the command palette binding and the new help overlay
• mount a ? / Shift+/ shortcuts overlay in AppShellLayout while ignoring editable fields
• document the shortcut registry, accessibility behavior, and usage in docs/KEYBOARD_SHORTCUTS.md

Closes #919

Verification

• pnpm exec vitest run src/components/shell/KeyboardShortcutsOverlay.test.tsx src/components/shell/AppShellLayout.test.tsx src/hooks/tests/useCommandPalette.test.ts
• pnpm exec eslint src/components/shell/AppShellLayout.tsx src/components/shell/AppShellLayout.test.tsx src/components/shell/KeyboardShortcutsOverlay.tsx src/components/shell/KeyboardShortcutsOverlay.test.tsx src/hooks/useCommandPalette.ts src/lib/shortcuts/shortcutRegistry.ts src/lib/shortcuts/index.ts
• git diff --check

Notes

• Full pnpm exec tsc --noEmit is currently blocked by pre-existing syntax errors in src/components/shell/SidebarSearch.test.tsx before it reaches this change.
• Local install/test verification was run on Node 24 with engine-strict=false because this machine does not have the repo-requested Node 20 runtime available; the repo engine settings were not changed.

[vercel]

Someone is attempting to deploy a commit to the 1nonly's projects Team on Vercel.

A member of the Team first needs to authorize it.