feat: add video and gif wallpaper support

[sumerchoudhary78]

Adds support for videos and animated GIFs as wallpapers, alongside the existing static images. They appear in the wallpaper picker and launcher, cross-fade in the same way images do, and drive the Material colour scheme.

Usage

Drop a video (.mp4, .webm, .mkv, .mov, .avi, .m4v) or a .gif into your wallpapers directory, or pick one from the wallpaper picker / launcher (>wallpaper). Nothing to configure.

Two optional runtime dependencies, only needed for video (gifs need neither):

• qt6-multimedia — plays the video. If missing, the wallpaper falls back to a static extracted frame instead of failing.
• ffmpeg — extracts a frame from the video for the colour scheme and picker thumbnails.

How it works

• modules/background/Wallpaper.qml used to hold two CachingImage slots that cross-fade. They're now generic slots that load the right renderer per file type: WallpaperVideo (VideoOutput + MediaPlayer), WallpaperAnimated (AnimatedImage for gifs), or WallpaperImage (the existing CachingImage, unchanged). The fade/scale transition is identical.
• The caelestia CLI generates the colour scheme but can't read videos, so for a video a representative frame is extracted with ffmpeg (1s in, to skip fade-from-black intros, falling back to the first frame), cached under ~/.cache/caelestia/wallpapers/frames/, and the CLI / colour analysis runs on that frame. Picker and launcher thumbnails reuse it; the launcher shows a movie icon for videos.
• FileSystemModel's Images filter dropped any file QImageReader couldn't read, which excluded videos. It now treats an explicit nameFilters match as authoritative, so the wallpaper scan can include video files (separate commit).

Performance / GPU

A playing video decodes continuously, so to keep idle cost down:

• Only one video/gif decodes at a time — the hidden cross-fade slot is unloaded once it finishes fading out.
• Playback pauses automatically when nothing can see it: a fullscreen window on that monitor, the monitor's DPMS off (screen asleep), game mode on, or the session locked. It resumes when the wallpaper is visible again.
• Static image wallpapers are unaffected — same code path as before.
• gifs decode on CPU via AnimatedImage and pause under the same conditions.

Breaking changes / side effects

• None for existing image users; static wallpapers behave exactly as before.
• The new deps are optional; without qt6-multimedia, videos degrade to a static frame.
• Adds a small frame cache under ~/.cache/caelestia/wallpapers/frames/.

Testing

Built the plugin and ran the shell on Hyprland. Verified:

• mp4 and webm videos set as wallpaper (via both the picker and the launcher) play and loop.
• Animated gifs play.
• The Material colour scheme is generated correctly from the auto-extracted video frame.
• Playback pauses on fullscreen, lock, DPMS-off and game mode, and falls back to a static frame when QtMultimedia isn't available.

qmlformat, qml-lint-conventions.py, clang-format and qmllint all pass.