feat(theme): add themes page to picture, manadeck, and presi apps

- Add dedicated /themes page with ThemePage component to all three apps
- Add theme variant dropdown with "Alle Themes" link in navigation
- Add keyboard shortcut 'T' for quick access to themes page
- Export PillNavElement type from shared-ui package

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

packages/shared-ui/src/index.ts

@@ -44,5 +44,6 @@ export type {
 	KeyboardShortcut,
 	PillNavItem,
 	PillDropdownItem,
+	PillNavElement,
 	PillNavigationProps,
 } from './navigation';