feat: fit-contain letterbox + smart queue refill

- Replace center-crop with fit-contain (no photo cropping)
- Letterbox bars filled with adjacent edge average color
- Queue fetches in batches until 50 usable assets
- Auto-refill queue on exhaustion, dedup via shown history
- Filter RAW files from asset selection

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

platformio.ini

@@ -23,6 +23,7 @@ lib_deps =
     mathieucarbou/ESP Async WebServer @ ^3.0.6
     bblanchon/ArduinoJson @ ^7.0.0
     ricmoo/QRCode @ ^0.0.1
+    bitbank2/JPEGDEC @ ^1.8.4
 lib_ignore =
     WebServer