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/bootstrap/Cargo.toml

@@ -19,3 +19,5 @@ anyhow.workspace = true
 tracing.workspace = true
 tracing-subscriber.workspace = true
 dotenvy.workspace = true
+chrono.workspace = true
+chrono-tz.workspace = true