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/splash_screen.cpp

@@ -1,6 +1,5 @@
 #include "splash_screen.h"
 #include "menu_stars.h"
-#include "i18n.h"
 #include <math.h>
 
 namespace SplashScreen {
@@ -132,30 +131,6 @@ void begin(TFT_eSPI& tft) {
     tft.setTextSize(1);
 }
 
-void showGreeting(TFT_eSPI& tft) {
-    // Ohne per NTP synchronisierte Uhrzeit koennen wir die Tageszeit nicht
-    // bestimmen - dann lieber nichts anzeigen als zu raten (gleiches Muster
-    // wie bei der Nachtdimmung in main.cpp::isNightDimHours()).
-    time_t now = time(nullptr);
-    if (now <= 8 * 3600 * 2) return;
-
-    struct tm tmNow;
-    localtime_r(&now, &tmNow);
-    int h = tmNow.tm_hour;
-
-    StringId greetingId;
-    if (h < 12) greetingId = StringId::SPLASH_GREETING_MORNING;
-    else if (h < 18) greetingId = StringId::SPLASH_GREETING_DAY;
-    else greetingId = StringId::SPLASH_GREETING_EVENING;
-
-    int16_t cx = tft.width() / 2;
-    tft.setTextDatum(MC_DATUM);
-    tft.setTextColor(TFT_DARKGREEN, TFT_BLACK);
-    tft.setTextSize(1);
-    tft.drawString(I18n::t(greetingId), cx, 84);
-    tft.setTextDatum(TL_DATUM);
-}
-
 void setStatusLine(TFT_eSPI& tft, uint8_t slot, const String& text, uint16_t color) {
     if (slot >= MAX_STATUS_LINES) return;