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_de.h

@@ -198,7 +198,7 @@ static const char* const I18N_DE[] = {
 
     "Logbuch / WebUI",
     "Logbuch / WebUI",
-    "Über diese Webseite kannst du das Flugbuch direkt im Browser ansehen, herunterladen und einzeln löschen. Im selben WLAN einfach im Browser aufrufen:",
+    "Diese Webseite zeigt oben ein Live-Radar mit allen aktuell sichtbaren Flugzeugen und darunter das Flugbuch zum Ansehen, Herunterladen und einzelnen Löschen direkt im Browser. Im selben WLAN einfach im Browser aufrufen:",
     "Keine WLAN-Verbindung - die Seite ist nur erreichbar, solange eine Netzwerkverbindung besteht.",
 
     "Wetter",