ci: add Arch package build and Gitea Actions publish workflow

Package YellowJacket for the Gitea Arch registry:

- packaging/arch/PKGBUILD — builds the Wails app from source via `go tool
  wails`; version derived from git (pkgver) so every build is monotonic.
  Source is overridable (YJ_GITURL/YJ_GITREF) for CI vs. manual release builds.
- packaging/arch/yellowjacket.desktop — application menu entry.
- .gitea/workflows/arch-package.yml — on push to main, builds the package in
  an archlinux container and uploads it to the Arch registry.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/arch/yellowjacket.desktop

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Type=Application
+Name=YellowJacket
+GenericName=Music Player
+Comment=Music how it was meant to bee
+Exec=yellowjacket
+Icon=yellowjacket
+Terminal=false
+Categories=AudioVideo;Audio;Player;GTK;
+MimeType=audio/mpeg;audio/flac;audio/ogg;audio/x-wav;
+StartupWMClass=yellowjacket
+Keywords=music;player;audio;library;