mod: Phase 2a - add mod settings, I18n strings, and main.cpp integration

CrossPointSettings: Add mod-specific enums and fields:
- Clock: CLOCK_FORMAT, CLOCK_SIZE, TIMEZONE, clockFormat, clockSize,
  timezone, timezoneOffsetHours, autoNtpSync
- Sleep: SLEEP_SCREEN_LETTERBOX_FILL, sleepScreenLetterboxFill
- Reader: preferredPortrait, preferredLandscape
- Indexing: INDEXING_DISPLAY, indexingDisplay
- getTimezonePosixStr() for POSIX TZ string generation

main.cpp: Integrate mod initialization:
- OPDS store loading, boot NTP sync, timezone application
- Clock refresh loop (re-render on minute change)
- RTC time logging on boot

SettingsList.h: Add clock, timezone, and letterbox fill settings
JsonSettingsIO.cpp: Handle int8_t timezoneOffsetHours separately
I18n: Add ~80 mod string keys (english.yaml + regenerated I18nKeys.h)

Made-with: Cursor

src/JsonSettingsIO.cpp

@@ -121,6 +121,9 @@ bool JsonSettingsIO::saveSettings(const CrossPointSettings& s, const char* path)
   doc["frontButtonLeft"] = s.frontButtonLeft;
   doc["frontButtonRight"] = s.frontButtonRight;
 
+  // Mod: timezone offset is int8_t, not uint8_t — handled separately
+  doc["timezoneOffsetHours"] = s.timezoneOffsetHours;
+
   String json;
   serializeJson(doc, json);
   return Storage.writeFile(path, json);
@@ -200,6 +203,11 @@ bool JsonSettingsIO::loadSettings(CrossPointSettings& s, const char* json, bool*
       clamp(doc["frontButtonRight"] | (uint8_t)S::FRONT_HW_RIGHT, S::FRONT_BUTTON_HARDWARE_COUNT, S::FRONT_HW_RIGHT);
   CrossPointSettings::validateFrontButtonMapping(s);
 
+  // Mod: timezone offset is int8_t, not uint8_t
+  s.timezoneOffsetHours = doc["timezoneOffsetHours"] | (int8_t)0;
+  if (s.timezoneOffsetHours < -12) s.timezoneOffsetHours = -12;
+  if (s.timezoneOffsetHours > 14) s.timezoneOffsetHours = 14;
+
   LOG_DBG("CPS", "Settings loaded from file");
 
   return true;