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/src/lib.rs

@@ -782,6 +782,27 @@ pub fn run() {
                 }
             })?;
 
+            // WebKitGTK ships `enable-media-stream`/`enable-webrtc` OFF by
+            // default (unlike WebView2/WKWebView), which leaves
+            // `navigator.mediaDevices` undefined and makes Element Call
+            // report "browser does not support WebRTC". Turn them on and
+            // auto-grant the resulting camera/mic permission prompt, mirroring
+            // the WebView2 handling above.
+            #[cfg(target_os = "linux")]
+            window.with_webview(|webview| {
+                use webkit2gtk::{PermissionRequestExt, SettingsExt, WebViewExt};
+
+                let wv = webview.inner();
+                if let Some(settings) = WebViewExt::settings(&wv) {
+                    settings.set_enable_media_stream(true);
+                    settings.set_enable_webrtc(true);
+                }
+                wv.connect_permission_request(|_webview, request| {
+                    request.allow();
+                    true
+                });
+            })?;
+
             // Native desktop feature modules (power/call-continuity, etc.).
             native::setup(app.handle())?;