refactor(store): Extract utility functions into store/utils/

Move pure utility functions from app-store.ts and type files into
dedicated utils modules for better separation of concerns:

- theme-utils.ts: Theme and font storage utilities
- shortcut-utils.ts: Keyboard shortcut parsing/formatting
- usage-utils.ts: Usage limit checking

All utilities are re-exported from store/utils/index.ts and
app-store.ts maintains backward compatibility for existing imports.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/ui/src/store/utils/index.ts

@@ -0,0 +1,13 @@
+// Theme utilities (PUBLIC)
+export {
+  THEME_STORAGE_KEY,
+  getStoredTheme,
+  getStoredFontSans,
+  getStoredFontMono,
+} from './theme-utils';
+
+// Shortcut utilities (PUBLIC)
+export { parseShortcut, formatShortcut, DEFAULT_KEYBOARD_SHORTCUTS } from './shortcut-utils';
+
+// Usage utilities (PUBLIC)
+export { isClaudeUsageAtLimit } from './usage-utils';