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/rss/src/lib.rs

@@ -28,7 +28,10 @@ impl DataSourcePort for RssAdapter {
     type Error = RssError;
 
     async fn poll(&self, source: &DataSource) -> Result<Value, Self::Error> {
-        let url = source.config.url.as_ref().ok_or(RssError::NoUrl)?;
+        let domain::DataSourceConfig::External { ref url, .. } = source.config else {
+            return Err(RssError::NoUrl);
+        };
+        let url = url.as_ref().ok_or(RssError::NoUrl)?;
 
         let resp = self
             .client