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/domain/tests/data_source_tests.rs

@@ -7,7 +7,7 @@ fn make_source(source_type: DataSourceType, url: Option<&str>, poll: Duration) -
         name: "test".into(),
         source_type,
         poll_interval: poll,
-        config: DataSourceConfig {
+        config: DataSourceConfig::External {
             url: url.map(Into::into),
             headers: vec![],
             api_key: None,