mod: add clock settings tab, timezone support, and clock size option

Fix clock persistence bug caused by stale legacy read in settings
deserialization. Add clock size setting (Small/Medium/Large) and
timezone selection with North American presets plus custom UTC offset.
Move all clock-related settings into a dedicated Clock tab, rename
"Home Screen Clock" to "Clock", and move minute-change detection to
main loop so the header clock updates on every screen.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/main.cpp

@@ -10,6 +10,7 @@
 #include <SPI.h>
 #include <builtinFonts/all.h>
 
+#include <cstdlib>
 #include <cstring>
 #include <ctime>
 
@@ -325,6 +326,11 @@ void setup() {
   }
 
   SETTINGS.loadFromFile();
+
+  // Apply saved timezone setting on boot
+  setenv("TZ", SETTINGS.getTimezonePosixStr(), 1);
+  tzset();
+
   I18N.loadSettings();
   KOREADER_STORE.loadFromFile();
   UITheme::getInstance().reload();
@@ -441,6 +447,20 @@ void loop() {
     return;
   }
 
+  // Refresh screen when the displayed minute changes (clock in header)
+  if (SETTINGS.clockFormat != CrossPointSettings::CLOCK_OFF && currentActivity) {
+    static int lastRenderedMinute = -1;
+    time_t now = time(nullptr);
+    struct tm* t = localtime(&now);
+    if (t != nullptr && t->tm_year > 100) {
+      const int currentMinute = t->tm_hour * 60 + t->tm_min;
+      if (lastRenderedMinute >= 0 && currentMinute != lastRenderedMinute) {
+        currentActivity->requestUpdate();
+      }
+      lastRenderedMinute = currentMinute;
+    }
+  }
+
   const unsigned long activityStartTime = millis();
   if (currentActivity) {
     currentActivity->loop();