feat: advertise raw GIF support to Beeper Desktop

[clins1994]

Summary

• advertise the dedicated fi.mau.gif room capability for raw image/gif
• route Beeper Desktop raw-GIF events (m.video with fi.mau.gif) through the existing LINE image/GIF send path
• intentionally exclude MP4 and WebM GIF variants so this first pass needs no ffmpeg conversion or shared media-path changes

Related to #90

Why this is intentionally small

The bridge already sends actual GIF files from Beeper mobile and ordinary media uploads. Beeper Desktop uses a separate GIF capability to decide whether its GIF picker can send to a room. Advertising only image/gif lets Desktop select the format the bridge already supports.

Video-backed GIFs remain ordinary videos and are not advertised as GIF-capable yet. They can be added later if live testing shows Desktop cannot provide a raw GIF.

Testing

• go test ./... -count=1
• go vet $(go list ./... | grep -v /ltsm)
• staticcheck $(go list ./... | grep -v /ltsm)
• go build ./...
• goimports -local github.com/highesttt/matrix-line-messenger
• git diff --check

Beeper Cloud verification

• Confirm the GIF picker is enabled in a LINE room on Beeper Desktop for macOS
• Send a GIF from Beeper Desktop to a LINE DM
• Confirm it arrives and animates in LINE
• Repeat in a LINE group