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.h

@@ -19,9 +19,6 @@ enum class StringId : uint8_t {
     SPLASH_WIFI_FAILED,
     SPLASH_GETTING_LOCATION,
     SPLASH_READY,
-    SPLASH_GREETING_MORNING,
-    SPLASH_GREETING_DAY,
-    SPLASH_GREETING_EVENING,
 
     SD_REQUIRED_LINE1,
     SD_REQUIRED_LINE2,
@@ -186,6 +183,19 @@ enum class StringId : uint8_t {
     AIRCRAFT_LIST_SORT_ALTITUDE,
     AIRCRAFT_LIST_SORT_CALLSIGN,
 
+    MENU_ABOUT,
+    ABOUT_DESC1,
+    ABOUT_DESC2,
+    ABOUT_VERSION_PREFIX,
+
+    MENU_BRIGHTNESS_PREFIX,
+    BRIGHTNESS_TITLE,
+
+    // Grosse Warn-Ueberlage beim Einschalten des Flugbuchs (Menue >
+    // Flugoptionen > Flugbuch), erklaert die 24h-Sicherheitsabschaltung.
+    MENU_LOGBOOK_WARNING_TITLE,
+    MENU_LOGBOOK_WARNING_BODY,
+
     COUNT
 };