v2.7.3: Flight logbook safety net, radar/clock/timezone fixes

New:
- Flight logbook safety net: disabled by default, requires confirming
  a full-screen warning before it can be turned on, and automatically
  switches off again after exactly 24 hours - survives restarts and
  power cycles (no valid enable timestamp = guaranteed off). Each
  activation now writes its own CSV file instead of endlessly
  appending to one growing file, and every file can be deleted
  individually from Logbook files.
- Adjustable display brightness (10-100% in 10% steps, Menu -> System
  -> "Brightness") with a live preview.
- About screen (Menu -> System -> "About") showing project name,
  description, and firmware version.

Fixed:
- Radar's red altitude band (>9100m/30000ft) no longer gets dimmed at
  night - the previous dim tone looked orange-brown and was hard to
  tell apart from yellow. Legend and markers are consistent again.
- Radius change no longer leaves a stale "tail" trail behind each
  aircraft between the old and new position.
- Splash screen no longer shows a time-of-day greeting that appeared
  too late in the boot sequence to be useful; the star animation now
  starts immediately instead of only near the end of boot.
- Clock stayed on raw UTC after the first boot instead of local time
  (missing DST handling in some timezones) because the location wait
  loop skipped the timezone lookup once a location was already saved.
- Header clock no longer leaves stale digit fragments behind the
  current time, most visible on the last minute digit.

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

src/i18n_fr.h

@@ -19,9 +19,6 @@ static const char* const I18N_FR[] = {
     "ÉCHEC WiFi",
     "Localisation...",
     "Prêt !",
-    "Bonjour",
-    "Bonjour",
-    "Bonsoir",
 
     "Pour cette appli une",
     "carte SD est",
@@ -186,6 +183,17 @@ static const char* const I18N_FR[] = {
     "Distance",
     "Altitude",
     "Indicatif",
+
+    "À propos",
+    "Radar de vol ADS-B en",
+    "direct pour l'ESP32 CYD.",
+    "Version : ",
+
+    "Luminosité : ",
+    "Luminosité",
+
+    "Attention !!!",
+    "Le journal de vol enregistre en continu tous les avions visibles sur la carte SD. Pour éviter qu'elle se remplisse sans que vous le remarquiez, il se désactive automatiquement au bout de 24 heures.",
 };
 
 static_assert(sizeof(I18N_FR) / sizeof(I18N_FR[0]) == (size_t)StringId::COUNT,