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/README.md

@@ -12,6 +12,10 @@ Uses the shared board library and LVGL 9.6 (`../shared`), NimBLE-Arduino 2.5.1.
   are listed first in green. **Tap one** to select it: it is remembered (NVS) and connected immediately.
   **Forget** clears the remembered adapter, **Done** (or swipe right) returns to the speed page.
 - **Swipe left again → settings** — screen rotation 0 / 90 / 180 / 270 degrees, and **screen off without data** Off / 2 / 5 / 10 min (default 2). Both remembered in NVS.
+- **Swipe left once more → background** — `Built-in` plus every GIF in `/gifs` on the SD card (size and
+  dimensions shown; not 360×360 in orange). Tap one: it is copied into the device's flash (progress shown) and
+  played from there, so the SD card is not needed afterwards. At boot the device checks (in the background)
+  whether the chosen file on the SD card changed (size/date) and re-copies it. **Rescan** re-reads the card.
 - **Idle** — after that time without speed data (ignition off, adapter gone) the screen goes dark, the GIF pauses and
   OBD polling stops; only `ATRV` every 30 s (answered by the adapter itself, doesn't wake the car's bus).
   Wakes on a touch (the touch is swallowed) or by itself when the voltage rises above 13.2 V (engine started).
@@ -31,6 +35,17 @@ Serial (115200) prints status changes as `[obd] ...`.
 | `src/main.cpp` | LVGL UI (tileview: speed page + menu page) |
 | `src/obd_link.cpp/.h` | BLE scan, connect/reconnect, ELM327 init and speed polling (background task on core 0) |
 | `src/font_speed_120.c` | Montserrat Medium 120 px, digits and `-`, 4 bpp (generated with `lv_font_conv`, see header) |
-| `data/bg.gif` | Background (`ezgif-5-cb56986bc7.gif`, 360×360), linked into the firmware via `board_build.embed_files` |
+| `src/bg_store.cpp/.h` | Background GIF from the SD card: list `/gifs`, copy into the `bggif` flash partition (header written last), mmap for playback, change check at boot (background task) |
+| `partitions.csv` | 4 MB app (no OTA) + 11.9 MB `bggif` raw area + NVS |
+| `data/bg.gif` | Built-in background (`ezgif-5-cb56986bc7.gif`, 360×360), linked into the firmware via `board_build.embed_files` |
+| `sd/gifs/` | (not in git) GIFs converted with `../tools/gif_to_360.py`, ready to copy to the SD card |
 
-Replace `data/bg.gif` with any 360×360 GIF to change the background (keep it under ~3 MB).
+## GIFs for the SD card
+Convert any GIF to 360×360 (scale to cover, center crop, frame timing kept) with
+```
+pip install pillow
+python ../tools/gif_to_360.py <gif files or folders> -o sd/gifs
+```
+and copy the files into a folder `gifs` on the SD card. Max ~11.5 MB per GIF. GIFs are played from flash at their own
+frame rate (360×360: ~10–12 ms decode per frame).
+Replace `data/bg.gif` to change the built-in background.