internal data sources (clock, static text), connection indicator, rendering fixes

DataSourceConfig refactored to enum: External/Clock/StaticText. Clock
generates formatted time via chrono, static text emits configured string.

ESP32: connection status indicator (green/red dot bottom-right), per-widget
clear before redraw, RenderEvent enum for local + server messages.

Polling uses DataUpdate instead of ScreenUpdate to avoid wiping widget state.
Empty mappings passthrough raw source data for internal sources.

crates/adapters/config-sqlite/tests/config_store_tests.rs

@@ -36,7 +36,7 @@ fn weather_source() -> DataSource {
         name: "openweather".into(),
         source_type: DataSourceType::Weather,
         poll_interval: Duration::from_secs(300),
-        config: DataSourceConfig {
+        config: DataSourceConfig::External {
             url: Some("https://api.openweather.org".into()),
             headers: vec![],
             api_key: Some("test-key".into()),
@@ -125,8 +125,13 @@ async fn save_and_retrieve_data_source() {
     assert_eq!(ds.name, "openweather");
     assert_eq!(ds.source_type, DataSourceType::Weather);
     assert_eq!(ds.poll_interval, Duration::from_secs(300));
-    assert_eq!(ds.config.url, Some("https://api.openweather.org".into()));
-    assert_eq!(ds.config.api_key, Some("test-key".into()));
+    match &ds.config {
+        DataSourceConfig::External { url, api_key, .. } => {
+            assert_eq!(*url, Some("https://api.openweather.org".into()));
+            assert_eq!(*api_key, Some("test-key".into()));
+        }
+        _ => panic!("expected External config"),
+    }
 }
 
 #[tokio::test]