feat: add keepalive ping, state restoration, and fix baud rate

Add keepalive ping (5s interval, 2s timeout) to detect unresponsive
dongles behind UART bridges where the serial port stays open across
ESP32 resets. On reconnect, saved radio config and RX state are
re-sent so connected clients continue working transparently.

Also fix serial port opened without baud rate (defaulted to 9600
instead of 115200), preventing connection to UART boards.

CHANGELOG.md

@@ -1,5 +1,20 @@
 # 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.
+
+### Fixed
+
+- Serial port opened without baud rate (defaulted to 9600 instead of 115200),
+  preventing connection to UART boards behind CP2102 bridges.
+
 ## 0.2.0 — 2026-04-08
 
 ### Features