feat: implement cross-platform features and UI integration

- iOS: Add BackgroundSyncService, SyncScheduler, SyncWorker, BookmarkViewModel, FeedViewModel
- iOS: Add BackgroundSyncService, SyncScheduler, SyncWorker services
- Linux: Add settings-store.vala, State.vala signals, view widgets (FeedList, FeedDetail, AddFeed, Search, Settings, Bookmark)
- Linux: Add bookmark-store.vala, bookmark vala model, search-service.vala
- Android: Add NotificationService, NotificationManager, NotificationPreferencesStore
- Android: Add BookmarkDao, BookmarkRepository, SettingsStore
- Add unit tests for iOS, Android, Linux
- Add integration tests
- Add performance benchmarks
- Update tasks and documentation

Co-Authored-By: Paperclip <noreply@paperclip.ing>

android/src/main/java/com/rssuper/search/SearchService.kt

@@ -14,18 +14,39 @@ class SearchService(
     private val searchHistoryDao: SearchHistoryDao,
     private val resultProvider: SearchResultProvider
 ) {
-    private val cache = mutableMapOf<String, List<SearchResult>>()
+    private data class CacheEntry(val results: List<SearchResult>, val timestamp: Long)
+    private val cache = mutableMapOf<String, CacheEntry>()
     private val maxCacheSize = 100
+    private val cacheExpirationMs = 5 * 60 * 1000L // 5 minutes
+    
+    private fun isCacheEntryExpired(entry: CacheEntry): Boolean {
+        return System.currentTimeMillis() - entry.timestamp > cacheExpirationMs
+    }
+    
+    private fun cleanExpiredCacheEntries() {
+        cache.keys.removeAll { key ->
+            cache[key]?.let { isCacheEntryExpired(it) } ?: false
+        }
+    }
     
     fun search(query: String): Flow<List<SearchResult>> {
         val cacheKey = query.hashCode().toString()
         
-        // Return cached results if available
-        cache[cacheKey]?.let { return flow { emit(it) } }
+        // Clean expired entries periodically
+        if (cache.size > maxCacheSize / 2) {
+            cleanExpiredCacheEntries()
+        }
+        
+        // Return cached results if available and not expired
+        cache[cacheKey]?.let { entry ->
+            if (!isCacheEntryExpired(entry)) {
+                return flow { emit(entry.results) }
+            }
+        }
         
         return flow {
             val results = resultProvider.search(query)
-            cache[cacheKey] = results
+            cache[cacheKey] = CacheEntry(results, System.currentTimeMillis())
             if (cache.size > maxCacheSize) {
                 cache.remove(cache.keys.first())
             }