PKGBUILD: update deps, drop config.toml install, fix desktop file

- Updated pkgdesc to match current multi-platform scope (YouTube, TikTok, Twitch, etc.)
- Added runtime deps: ffmpeg, xdg-utils
- Added optdepends: libnotify for desktop notifications
- Added pkgver() function for VCS auto-versioning
- Switched --locked to --frozen (stricter, requires Cargo.lock)
- Dropped config.toml install (gitignored, app generates on first launch)
- Removed conditional icon check (file always exists)
- Added README.md install

youtube-backup.desktop: fixed Exec/Icon names to match binary (yt-offline)
- Added GenericName, Keywords, proper AudioVideo;Video;Network; categories

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

youtube-backup.desktop

@@ -1,9 +1,11 @@
 [Desktop Entry]
 Type=Application
-Name=YouTube Offline
-Comment=Download and manage YouTube channel backups
-Exec=youtube-backup
-Icon=youtube-backup
-Categories=Utility;Video;
+Name=YT Offline
+GenericName=Video Archiver
+Comment=Self-hosted archive for YouTube, TikTok, Twitch, and more
+Exec=yt-offline
+Icon=yt-offline
+Categories=AudioVideo;Video;Network;
+Keywords=youtube;yt-dlp;archive;backup;download;
 StartupNotify=true
 Terminal=false