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/media/tests/media_tests.rs

@@ -45,7 +45,7 @@ fn make_source(url: String) -> DataSource {
         name: "navidrome".into(),
         source_type: DataSourceType::Media,
         poll_interval: Duration::from_secs(5),
-        config: DataSourceConfig {
+        config: DataSourceConfig::External {
             url: Some(url),
             headers: vec![
                 ("username".into(), "test".into()),