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

platformio.ini

@@ -72,6 +72,22 @@ build_flags =
   -DLOG_LEVEL=2 ; Set log level to debug for development builds
 
 
+[env:mod]
+extends = base
+extra_scripts =
+  ${base.extra_scripts}
+  pre:scripts/inject_mod_version.py
+build_flags =
+  ${base.build_flags}
+  -DOMIT_OPENDYSLEXIC
+  -DOMIT_HYPH_DE
+  -DOMIT_HYPH_ES
+  -DOMIT_HYPH_FR
+  -DOMIT_HYPH_IT
+  -DOMIT_HYPH_RU
+  -DENABLE_SERIAL_LOG
+  -DLOG_LEVEL=2 ; Set log level to debug for mod builds
+
 [env:gh_release]
 extends = base
 build_flags =