OTA close-WebUI hint, OTA diagnostic logging, WebUI radar improvements

The OTA confirmation dialog now warns to close the WebUI page first -
its 3-second polling was interfering with the firmware download.

performUpdate() now logs progress and, on failure, the actual
HTTPUpdate error code/string over serial instead of just returning
false, to make future OTA failures easier to diagnose.

WebUI live radar: all four compass directions (N/E/S/W) instead of
just N, clickable aircraft with a persistent info panel (callsign,
altitude, speed, distance/bearing, heading, squawk), a client-side
range selector (10/25/50/100 km, independent of the device setting),
and twinkling background stars outside the radar circle redrawn at
150ms independent of the 3s aircraft-data poll.

Note: the "notable" (orange ring) field in /radar.json remains
hardcoded false - Config::NOTABLE_CALLSIGN_PREFIXES still doesn't
exist anywhere in this project.

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

src/i18n_en.h

@@ -247,7 +247,7 @@ static const char* const I18N_EN[] = {
     "Check failed. Check your WiFi connection.",
     "Already up to date: v",
     "Update available: v",
-    "A new firmware update will be downloaded and installed. Please don't interrupt the WiFi connection or power supply during this process. You'll be asked to restart once installation is complete.",
+    "A new firmware update will be downloaded and installed. Please don't interrupt the WiFi connection or power supply during this process. You'll be asked to restart once installation is complete. If the WebUI page (logbook/radar) is open in a browser, please close it first - it regularly polls the device for data and can interfere with the download otherwise.",
     "Installing update ",
     "Update failed",
     "Update installed",