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>

.gitignore

@@ -4,3 +4,6 @@
 .vscode/c_cpp_properties.json
 .vscode/launch.json
 .vscode/ipch
+
+# generated by tools/gif_to_360.py (copy to the SD card, regenerate any time)
+12-OBD-Speed/sd/