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-api/tests/api_tests.rs

@@ -171,9 +171,7 @@ async fn create_and_get_data_source() {
         "name": "weather_api",
         "source_type": "weather",
         "poll_interval_secs": 300,
-        "url": "https://api.openweather.org",
-        "api_key": "test-key",
-        "headers": []
+        "config": {"type": "external", "url": "https://api.openweather.org", "api_key": "test-key", "headers": []}
     }"#;
 
     let resp = app