state recovery, polling optimizations, error rendering

widget states cached to SQLite, loaded on startup to seed DataProjection
so server restart preserves last-known data for reconnecting clients.

polling: first poll runs immediately, widget list cached per-task with
30s refresh, static text polled once inline instead of looping.

poll failures propagate WidgetError::SourceUnavailable to clients.
render engine prepends [offline] prefix in accent color, stale data
preserved below.

crates/application/tests/support/mod.rs

@@ -1,6 +1,6 @@
 use domain::{
     ConfigRepository, DataSource, DataSourceId, DomainEvent, EventPublisher, Layout, LayoutPreset,
-    LayoutPresetId, ThemeConfig, User, WidgetConfig, WidgetId,
+    LayoutPresetId, ThemeConfig, User, WidgetConfig, WidgetId, WidgetState,
 };
 use std::collections::HashMap;
 use std::sync::Mutex;
@@ -135,6 +135,17 @@ impl ConfigRepository for InMemoryConfigRepository {
     async fn count_users(&self) -> Result<u32, Self::Error> {
         Ok(0)
     }
+
+    async fn save_widget_states(
+        &self,
+        _states: &[(WidgetId, WidgetState)],
+    ) -> Result<(), Self::Error> {
+        Ok(())
+    }
+
+    async fn load_widget_states(&self) -> Result<Vec<(WidgetId, WidgetState)>, Self::Error> {
+        Ok(vec![])
+    }
 }
 
 pub struct InMemoryEventPublisher {