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/main.rs

@@ -50,7 +50,7 @@ fn run_station(
     info!("Connecting WiFi...");
     match hal::wifi::init(modem, sysloop.clone(), nvs.clone(), &cfg.wifi_ssid, &cfg.wifi_pass) {
         Ok(_wifi) => {
-            let (tx, rx) = mpsc::channel();
+            let (tx, rx) = mpsc::channel::<tasks::RenderEvent>();
             tasks::network::spawn(cfg.server_addr, tx);
             tasks::render::run(config::SCREEN, display, rx);
         }