feat: add keepalive ping and state restoration after dongle reconnect

UART boards (CP2102 bridge) keep the serial port alive across ESP32 resets,
so the mux never detected the board rebooting. Now the writer sends a
keepalive Ping every 5s of idle and declares the dongle lost after a
2s timeout.

On reconnect, saved radio config and RX state are re-sent to the dongle
so connected clients continue working without re-establishing state.

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Changelog
 
+## Unreleased
+
+### Added
+
+- Keepalive ping: detects unresponsive dongles (e.g. UART boards behind a
+  CP2102 bridge where the serial port stays open across ESP32 resets). Pings
+  every 5 seconds of idle, declares dongle lost after 2-second timeout.
+- Radio state restoration after reconnect: saved SetConfig and StartRx are
+  re-sent to the dongle so clients don't need to know a reset happened.
+
 ## 0.2.2 — 2026-04-08
 
 ### Fixed