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/src/entities/widget_config.rs

@@ -35,6 +35,15 @@ impl WidgetConfig {
     }
 
     pub fn extract(&self, raw: &Value) -> WidgetState {
+        let has_mappings = self.mappings.iter().any(|m| !m.source_path.is_empty());
+        if !has_mappings {
+            let data = match raw {
+                Value::Object(map) => map.clone(),
+                _ => BTreeMap::new(),
+            };
+            return WidgetState { data, error: None };
+        }
+
         let budget = self.max_data_size as usize;
         let mut used = 0usize;
         let mut data = BTreeMap::new();