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/config.h

@@ -2,6 +2,25 @@
 #include <Arduino.h>
 
 namespace Config {
+    // Wird bei jedem Versions-Release von Karl aktualisiert (siehe
+    // CLAUDE.md-Workflow "Standard-Workflow: Push & Release") - erscheint
+    // im Info-Screen (Menue > System > Info) und muss zum jeweiligen
+    // Git-Tag passen.
+    constexpr const char* APP_VERSION = "2.7.3";
+
+    // Display-Helligkeit (Menue > System > Helligkeit), in Prozent.
+    // MIN bewusst nicht 0 - ein komplett dunkles Display koennte sonst wie
+    // ein Defekt wirken statt wie eine Einstellung.
+    constexpr uint8_t BRIGHTNESS_MIN_PERCENT = 10;
+    constexpr uint8_t BRIGHTNESS_MAX_PERCENT = 100;
+    constexpr uint8_t BRIGHTNESS_STEP_PERCENT = 10;
+
+    // Nachtmodus (22-6 Uhr) dimmt relativ zur jeweils eingestellten normalen
+    // Helligkeit, nicht auf einen festen Absolutwert - sonst waere der
+    // Dimm-Effekt bei niedrig eingestellter Normalhelligkeit wirkungslos
+    // oder wuerde das Display nachts sogar heller machen als tagsueber.
+    constexpr uint8_t NIGHT_DIM_REDUCTION_PERCENT = 40;
+
     constexpr const char* IP_GEO_HOST = "ip-api.com";
     constexpr const char* IP_GEO_PATH = "/json/?fields=status,lat,lon,offset,countryCode";