AP-057: Add command palette (Cmd/Ctrl+K) with shared entity search hook

[navyamidha95339-arch]

Summary
Adds a global command palette to the Admin Panel, opened with Cmd+K / Ctrl+K from any page, for jumping between pages and searching across users, shifts, and incidents without leaving the keyboard.

Checklist

✅ Shared search hook: extracted the existing search logic from GlobalSearch.jsx into a new useEntitySearch.js hook, so GlobalSearch and CommandPalette share one implementation instead of duplicating it
✅ Incidents added to search scope: since the hook is shared, incidents now show up in both the command palette and the existing global search
✅ Jump-to-page entries: palette lists admin panel pages so admins can navigate directly by typing a page name
✅ Full keyboard navigation: ↑/↓ to move through results, Enter to select, Escape to close
✅ Mounted globally in AdminLayout.jsx, so the shortcut works on every admin page
✅ No regression in GlobalSearch: same behaviour as before apart from the added incident results
✅ Verified end-to-end locally: open → search → navigate → close

Evidence
Screenshots attached: palette open on an admin page, search results across users/shifts/incidents, and keyboard selection highlighted.

[Krisha190235]

Rebased onto current main. Kept useEntitySearch as the single source of search logic (superseding #644's inline helpers — GlobalSearch and CommandPalette both consume it), and wired GlobalSearch + CommandPalette to useTheme so they respect dark mode (#634). Resolved the AdminLayout clash by keeping both ViewAsBanner and CommandPalette, and wrapped the GlobalSearch test in renderWithTheme. Net diff is 5 files; eslint clean, full admin suite green (19 suites / 72 tests), build passes. Force‑pushed to update in place. 🙏