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/http-json/src/lib.rs

@@ -47,15 +47,23 @@ impl DataSourcePort for HttpJsonAdapter {
     type Error = HttpJsonError;
 
     async fn poll(&self, source: &DataSource) -> Result<Value, Self::Error> {
-        let url = source.config.url.as_ref().ok_or(HttpJsonError::NoUrl)?;
+        let domain::DataSourceConfig::External {
+            ref url,
+            ref headers,
+            ref api_key,
+        } = source.config
+        else {
+            return Err(HttpJsonError::NoUrl);
+        };
+        let url = url.as_ref().ok_or(HttpJsonError::NoUrl)?;
 
         let mut req = self.client.get(url);
 
-        for (key, val) in &source.config.headers {
+        for (key, val) in headers {
             req = req.header(key, val);
         }
 
-        if let Some(api_key) = &source.config.api_key {
+        if let Some(api_key) = api_key {
             req = req.header("Authorization", format!("Bearer {api_key}"));
         }