feat: Implement link health monitoring system with periodic checks and dead link detection

- Add LinkHealthCheckWorker for automated bi-daily link validation (12-hour intervals)
- Extend LinkEntity with isDeadLink and lastHealthCheck fields for tracking link status
- Create LinkDao queries for dead link retrieval and health status updates
- Implement getDeadLinksStream() in LinkRepository for paginated dead link viewing
- Add GeminiRepository.generateTags() method with model fallback strategy and

app/src/main/java/com/shaarit/presentation/nav/NavGraph.kt

@@ -42,6 +42,7 @@ sealed class Screen(val route: String) {
     object Dashboard : Screen("dashboard")
     object Settings : Screen("settings")
     object Help : Screen("help")
+    object DeadLinks : Screen("dead_links")
 }
 
 @Composable
@@ -125,6 +126,7 @@ fun AppNavGraph(
                 onNavigateToSettings = { navController.navigate(Screen.Settings.route) },
                 onNavigateToRandom = { },
                 onNavigateToHelp = { navController.navigate(Screen.Help.route) },
+                onNavigateToDeadLinks = { navController.navigate(Screen.DeadLinks.route) },
                 initialTagFilter = tag,
                 initialCollectionId = collectionId
             )
@@ -256,5 +258,16 @@ fun AppNavGraph(
                 onNavigateBack = { navController.popBackStack() }
             )
         }
+
+        composable(
+            route = Screen.DeadLinks.route
+        ) {
+            com.shaarit.presentation.deadlinks.DeadLinksScreen(
+                onNavigateBack = { navController.popBackStack() },
+                onNavigateToEdit = { linkId ->
+                    navController.navigate(Screen.Edit.createRoute(linkId))
+                }
+            )
+        }
     }
 }