Add WebUI live radar; mutex-protect AirlineFilter/AircraftWatchlist

The WebUI home page now shows a live radar canvas above the flight
logbook, polling a new /radar.json endpoint every 3 seconds. It
mirrors the device display: distance rings, aircraft colored by
altitude, ground vehicles as squares, rotorcraft as diamonds, and
emergency/watchlist/heavy-category rings in that priority. Applies
the same range, hide-ground-vehicles, and airline filters as the
device radar.

AirlineFilter and AircraftWatchlist gain proper mutex protection
around their in-memory arrays (previously only guarded around SD
file I/O) since the new /lists WebUI pages reach them from Core 0
(NetTask) in addition to their existing Core 1 (menu screen)
callers.

Note: the "notable aircraft" military/government callsign-prefix
marker is not implemented - Config::NOTABLE_CALLSIGN_PREFIXES was
never defined anywhere in this project, on the device display or
here. The "notable" ring currently fires on heavy-category aircraft
only.

Also documents the OTA-testing exception to the auto-flash rule in
CLAUDE.md (build-only during Push & Release, so the test device stays
on the previous version for OTA testing).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/i18n_en.h

@@ -199,7 +199,7 @@ static const char* const I18N_EN[] = {
 
     "Logbook / WebUI",
     "Logbook / WebUI",
-    "This web page lets you view, download and individually delete the flight logbook right from a browser. While connected to the same WiFi network, open a browser and go to:",
+    "This web page shows a live radar with all currently visible aircraft at the top, and below it lets you view, download and individually delete the flight logbook right from a browser. While connected to the same WiFi network, open a browser and go to:",
     "No WiFi connection - the page is only reachable while connected to a network.",
 
     "Weather",