feat: add route persistence and page visibility configuration

- Persist current route to Chrome Storage, restore on popup reopen
- Add page visibility configuration to control which pages are shown
- Update StorageSchema with new storage keys (app/currentRoute, app/visiblePages)
- Add PageType type and PAGE_CONFIG for type-safe page management
- Add navigation button styles with active state and hover effects
- Use loading state to prevent UI flicker during async data load

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

types/storage.d.ts

@@ -1,4 +1,8 @@
+export type PageType = 'timestamp' | 'storageCleaner';
+
 export interface StorageSchema {
+  'app/currentRoute': PageType;
+  'app/visiblePages': PageType[];
   'app/lastRoute': string;
   'app/theme': string;
   'storageCleaner/preferences': StorageCleanerPreferences;