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/stats_history_screen.cpp

@@ -136,7 +136,10 @@ void run(TFT_eSPI& tft) {
     tft.print(I18n::t(StringId::LOADING));
 
     FlightLogbook::DayEntry days[MAX_DAYS_QUERIED];
-    uint8_t count = FlightLogbook::listDays(days, MAX_DAYS_QUERIED);
+    // listDaySummaries() statt listDays(): mehrere Sitzungs-Dateien am
+    // selben Kalendertag (z.B. nach erneutem Einschalten) sollen hier
+    // weiterhin als EIN Balken pro Tag zaehlen statt als mehrere.
+    uint8_t count = FlightLogbook::listDaySummaries(days, MAX_DAYS_QUERIED);
 
     uint8_t barCount = (count > MAX_BARS) ? MAX_BARS : count;
     uint8_t startIdx = count - barCount;