background: extensive animated image support

[evencewang]

Summary

• This PR relates to its shell counterpart for supporting animated images (gif) as wallpapers: background: extensive animated image support shell#889
• src/caelestia/utils/wallpaper.py:
  • Now accepts GIFs, quick check to normalize file suffix
  • Added animated image metadata extraction, new helper _load_img_or_first_frame_in_rgb to safely read regular static image into RGB, or first frame animated image. flattens transparency, handles EXIF orientation, etc.
  • Uses img.copy(), nothing tied to open file handle.
  • Functions like check_wall and get_thumb now uses the helper.
• Little return type update for get_colours_for_wall
• set_wallpaper reuses animated image metadata, static image pipeline stays the same.
• And a quick fix to src/caelestia/parser for proper return type annotation and marking --threshold option as float.

Testing

Is done alongside the shell PR together, check that PR for more details