Copy in-use resources from SPI flash to built-in flash

Since reading files from the SPI flash is very slow we could copy the files that the current watchface needs into the internal flash, then use them from there instead of the flash. This could be implemented as an LVGL file system, although I don't think we need to use LittleFS and could instead use a trivial flat file system structure.

For example, when the casio watchface is selected the three font files it uses will be copied into the internal flash, and when the watchface is shown it will load the files from there much quicker.

Alternatively, we could implement a sort of caching LVGL file system that sits in between the LittleFS file system and the LVGL adapter that caches some files in the internal flash, but that might be trickier due to fragmentation.

Start by tracing the LVGL file-system adapter, LittleFS integration, and watchface resource-loading path. Compare the proposed per-watchface copy approach with an intermediate cache, then define which files are copied, where they reside in internal flash, and how they are invalidated. Done means the selected watchface loads its resources from internal flash without breaking other watchfaces.