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/client-esp32/src/tasks/mod.rs

@@ -1,2 +1,9 @@
 pub mod network;
 pub mod render;
+
+use protocol::ServerMessage;
+
+pub enum RenderEvent {
+    Server(ServerMessage),
+    ConnectionStatus(bool),
+}