feat: add cmd+k hotkey to focus search bar with kbd badge indicator

[pablaber]

Summary

• Installs @tanstack/react-hotkeys and wraps the app with HotkeysProvider in main.tsx
• Registers a global Mod+K hotkey (⌘K on Mac, Ctrl+K on Windows/Linux) that focuses the search input from anywhere on the page
• Adds a styled <kbd> badge on the right side of the search input showing the platform-appropriate shortcut hint; the badge fades out when the input is focused or has a query

Testing

5 new tests added to SearchBar.test.tsx:

Test	What it covers
renders the kbd badge when enabled	<kbd> element is present in the DOM when SearchBar is enabled
kbd badge has hidden class when input is focused	Badge gets search-bar-kbd--hidden class on focus
kbd badge has hidden class when query is non-empty	Badge gets search-bar-kbd--hidden class when user has typed
kbd badge does not have hidden class when input is blurred and query is empty	Badge is visible when idle
Mod+K focuses the search input	Firing Ctrl+K (jsdom resolves Mod → Control on Linux) moves document.activeElement to the input

All 89 tests pass (npm run lint && npm run test && npm run build clean).