12-OBD-Speed: choose the background GIF from the SD card

- background page (4th tile): built-in or any GIF in /gifs on the SD card
- chosen GIF is copied into a raw 11.9 MB 'bggif' flash partition (header
  written last) and played memory-mapped from flash; SD only needed when
  choosing or when the file changed (checked at boot in the background)
- own partitions.csv: 4 MB app + bggif + NVS
- tools/gif_to_360.py: convert GIFs to 360x360 keeping frame timing;
  converted output (12-OBD-Speed/sd/) is git-ignored

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

12-OBD-Speed/platformio.ini

@@ -9,8 +9,8 @@ dir = ../shared
 
 [env:jc3636w518]
 extends = jc3636w518
-; firmware + 1.6 MB background GIF does not fit huge_app (3 MB): 6.25 MB app slots
-board_build.partitions = default_16MB.csv
+; 4 MB app (no OTA) + 11.9 MB raw 'bggif' area for the background GIF chosen from the SD card
+board_build.partitions = partitions.csv
 ; background GIF linked into the firmware as-is (symbols _binary_data_bg_gif_start/_end)
 board_build.embed_files = data/bg.gif
 lib_deps =