mod: Phase 2c-e — GfxRenderer, themes, SleepActivity, SettingsActivity, platformio

- Add drawPixelGray to GfxRenderer for letterbox fill rendering
- Add PRERENDER_THUMB_HEIGHTS to UITheme for placeholder cover generation
- Add [env:mod] build environment to platformio.ini
- Implement sleep screen letterbox fill (solid/dithered) with edge
  caching in SleepActivity, including placeholder cover fallback
- Add Clock settings category to SettingsActivity with timezone,
  NTP sync, and set-time actions; replace CalibreSettings with
  OpdsServerListActivity; add DynamicEnum rendering support
- Add long-press book management to RecentBooksActivity

Made-with: Cursor

lib/GfxRenderer/GfxRenderer.h

@@ -83,6 +83,7 @@ class GfxRenderer {
 
   // Drawing
   void drawPixel(int x, int y, bool state = true) const;
+  void drawPixelGray(int x, int y, uint8_t val2bit) const;
   void drawLine(int x1, int y1, int x2, int y2, bool state = true) const;
   void drawLine(int x1, int y1, int x2, int y2, int lineWidth, bool state) const;
   void drawArc(int maxRadius, int cx, int cy, int xDir, int yDir, int lineWidth, bool state) const;