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/activities/home/HomeActivity.h

@@ -16,7 +16,6 @@ class HomeActivity final : public Activity {
   bool recentsLoaded = false;
   bool firstRenderDone = false;
   bool hasOpdsUrl = false;
-  int lastRenderedMinute = -1;     // Track displayed minute for clock auto-update
   bool coverRendered = false;      // Track if cover has been rendered once
   bool coverBufferStored = false;  // Track if cover buffer is stored
   uint8_t* coverBuffer = nullptr;  // HomeActivity's own buffer for cover image