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/application/tests/config_service_tests.rs

@@ -47,7 +47,7 @@ async fn create_data_source_rejects_invalid() {
         name: "bad".into(),
         source_type: DataSourceType::HttpJson,
         poll_interval: Duration::from_secs(60),
-        config: DataSourceConfig {
+        config: DataSourceConfig::External {
             url: None,
             headers: vec![],
             api_key: None,
@@ -70,7 +70,7 @@ async fn create_data_source_persists_valid_and_emits_event() {
         name: "weather".into(),
         source_type: DataSourceType::Weather,
         poll_interval: Duration::from_secs(300),
-        config: DataSourceConfig {
+        config: DataSourceConfig::External {
             url: Some("https://api.weather.com".into()),
             headers: vec![],
             api_key: None,