fix(linux): enable WebRTC in WebKitGTK; add Arch package build

WebKitGTK ships enable-media-stream/enable-webrtc off by default
(unlike WebView2/WKWebView), leaving navigator.mediaDevices undefined
and Element Call reporting "browser does not support WebRTC" on every
Linux build. Enable both settings and auto-grant the resulting
camera/mic permission request, mirroring the existing WebView2 handling.

Also add a build-arch CI job that repackages the existing .deb into a
real .pkg.tar.zst and uploads it to the Gitea release alongside the
Windows/Linux artifacts, for Arch/CachyOS users who'd rather use pacman
than an AppImage.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -43,6 +43,9 @@ tauri-plugin-autostart = "2" # P6-1 launch-on-login
 # default `blocking-api` feature and the default `async-io` runtime, so plain
 # default features suffice (no tokio integration needed here).
 zbus = "5"
+# Same version wry/tauri already pull in transitively (see Cargo.lock) — pinning
+# it directly lets us reach WebKitSettings/permission APIs wry doesn't expose.
+webkit2gtk = "2.0"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 webview2-com = "0.38"