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_de.h

@@ -19,9 +19,6 @@ static const char* const I18N_DE[] = {
     "WLAN FEHLGESCHLAGEN",
     "Ermittle Standort...",
     "Bereit!",
-    "Guten Morgen",
-    "Guten Tag",
-    "Guten Abend",
 
     "Für diese App wird",
     "eine SD-Karte",
@@ -186,6 +183,17 @@ static const char* const I18N_DE[] = {
     "Distanz",
     "Höhe",
     "Rufzeichen",
+
+    "Info",
+    "Live-ADS-B-Flugradar für",
+    "das ESP32 CYD-Display.",
+    "Version: ",
+
+    "Helligkeit: ",
+    "Helligkeit",
+
+    "Achtung!!!",
+    "Das Flugbuch speichert dauerhaft alle sichtbaren Flugzeuge auf der SD-Karte. Damit sie sich nicht unbemerkt füllt und die Logbuch-Menüs nicht langsam werden, schaltet es sich nach 24 Stunden automatisch wieder aus.",
 };
 
 static_assert(sizeof(I18N_DE) / sizeof(I18N_DE[0]) == (size_t)StringId::COUNT,