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/domain/src/ports/config_repository.rs

@@ -1,7 +1,7 @@
 use crate::entities::{
     DataSource, DataSourceId, LayoutPreset, LayoutPresetId, User, WidgetConfig, WidgetId,
 };
-use crate::value_objects::{Layout, ThemeConfig};
+use crate::value_objects::{Layout, ThemeConfig, WidgetState};
 use std::future::Future;
 
 pub trait ConfigRepository {
@@ -63,4 +63,12 @@ pub trait ConfigRepository {
     ) -> impl Future<Output = Result<Option<User>, Self::Error>> + Send;
     fn save_user(&self, user: &User) -> impl Future<Output = Result<(), Self::Error>> + Send;
     fn count_users(&self) -> impl Future<Output = Result<u32, Self::Error>> + Send;
+
+    fn save_widget_states(
+        &self,
+        states: &[(WidgetId, WidgetState)],
+    ) -> impl Future<Output = Result<(), Self::Error>> + Send;
+    fn load_widget_states(
+        &self,
+    ) -> impl Future<Output = Result<Vec<(WidgetId, WidgetState)>, Self::Error>> + Send;
 }