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/brightness_screen.cpp

@@ -0,0 +1,92 @@
+#include "brightness_screen.h"
+#include "settings_store.h"
+#include "touch_input.h"
+#include "menu_stars.h"
+#include "config.h"
+#include "i18n.h"
+
+namespace BrightnessScreen {
+
+namespace {
+    struct Rect {
+        int16_t x, y, w, h;
+        bool contains(int16_t px, int16_t py) const {
+            return px >= x && px < x + w && py >= y && py < y + h;
+        }
+    };
+
+    void drawButton(TFT_eSPI& tft, const Rect& r, const String& label) {
+        tft.fillRoundRect(r.x, r.y, r.w, r.h, 4, TFT_BLACK);
+        tft.drawRoundRect(r.x, r.y, r.w, r.h, 4, TFT_GREEN);
+        tft.setTextDatum(MC_DATUM);
+        tft.setTextColor(TFT_GREEN, TFT_BLACK);
+        tft.drawString(label, r.x + r.w / 2, r.y + r.h / 2);
+        tft.setTextDatum(TL_DATUM);
+    }
+
+    // Gleicher PWM-Kanal wie in main.cpp (dort in setup() mit ledcSetup()
+    // initialisiert) - hier bewusst als eigene Konstante dupliziert statt
+    // geteilt, siehe CLAUDE.md-Konvention ("jeder Screen unabhaengig").
+    constexpr uint8_t BACKLIGHT_PWM_CHANNEL = 0;
+
+    void applyLive(uint8_t percent) {
+        uint8_t pwm = (uint8_t)((uint16_t)percent * 255 / 100);
+        ledcWrite(BACKLIGHT_PWM_CHANNEL, pwm);
+    }
+}
+
+void run(TFT_eSPI& tft) {
+    bool done = false;
+    MenuStars::reset();
+
+    while (!done) {
+        uint8_t percent = SettingsStore::brightnessPercent();
+
+        tft.fillScreen(TFT_BLACK);
+        tft.setTextColor(TFT_GREEN, TFT_BLACK);
+        tft.setCursor(10, 14);
+        tft.println(I18n::t(StringId::BRIGHTNESS_TITLE));
+
+        char buf[8];
+        snprintf(buf, sizeof(buf), "%d%%", percent);
+        tft.setTextDatum(MC_DATUM);
+        tft.setTextSize(3);
+        tft.drawString(buf, Config::SCREEN_WIDTH / 2, 130);
+        tft.setTextSize(1);
+        tft.setTextDatum(TL_DATUM);
+
+        Rect minusBtn = {10, 190, 100, 60};
+        Rect plusBtn  = {(int16_t)(Config::SCREEN_WIDTH - 110), 190, 100, 60};
+        drawButton(tft, minusBtn, "-");
+        drawButton(tft, plusBtn, "+");
+
+        Rect backBtn = {10, (int16_t)(Config::SCREEN_HEIGHT - 50),
+                         (int16_t)(Config::SCREEN_WIDTH - 20), 40};
+        drawButton(tft, backBtn, I18n::t(StringId::BACK));
+
+        TouchInput::Point tap;
+        while (true) {
+            if (TouchInput::wasTapped(tap)) break;
+            MenuStars::update(tft);
+            delay(20);
+        }
+
+        if (minusBtn.contains(tap.x, tap.y)) {
+            uint8_t next = (percent > Config::BRIGHTNESS_MIN_PERCENT + Config::BRIGHTNESS_STEP_PERCENT - 1)
+                ? (uint8_t)(percent - Config::BRIGHTNESS_STEP_PERCENT)
+                : Config::BRIGHTNESS_MIN_PERCENT;
+            SettingsStore::setBrightnessPercent(next);
+            applyLive(next);
+        } else if (plusBtn.contains(tap.x, tap.y)) {
+            uint8_t next = (percent + Config::BRIGHTNESS_STEP_PERCENT <= Config::BRIGHTNESS_MAX_PERCENT)
+                ? (uint8_t)(percent + Config::BRIGHTNESS_STEP_PERCENT)
+                : Config::BRIGHTNESS_MAX_PERCENT;
+            SettingsStore::setBrightnessPercent(next);
+            applyLive(next);
+        } else if (backBtn.contains(tap.x, tap.y)) {
+            done = true;
+        }
+    }
+}
+
+}