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>

packaging/pacman/PKGBUILD

@@ -0,0 +1,39 @@
+# Built and published by CI (.gitea/workflows/release.yml, build-arch job) —
+# not published to AUR, just a real .pkg.tar.zst uploaded to the Gitea
+# release alongside the .deb/AppImage/exe. CI sed-patches pkgver below to the
+# release version before building.
+#
+# Repackages the .deb the same CI already builds (build-linux) rather than
+# recompiling from source: the web client needs the private @lotusguild/*
+# npm registry, and build-linux already produced a working Linux binary —
+# no reason to build it twice.
+pkgname=lotus-chat
+pkgver=0.0.0
+pkgrel=1
+pkgdesc="Matrix chat client for Lotus Guild (desktop app)"
+arch=('x86_64')
+url="https://chat.lotusguild.org"
+license=('AGPL-3.0-only')
+depends=(
+  'webkit2gtk-4.1'
+  'gtk3'
+  'libayatana-appindicator'
+  # WebRTC calling: WebKitGTK's media pipeline is backed by GStreamer, and
+  # these are the plugin sets that actually carry audio/video codecs.
+  'gst-plugins-good'
+  'gst-plugins-bad'
+  'gst-plugins-ugly'
+  'gst-libav'
+)
+provides=('lotus-chat')
+conflicts=('lotus-chat')
+options=('!strip')
+source=("LotusChat-x86_64.deb::https://code.lotusguild.org/LotusGuild/cinny-desktop/releases/download/latest/LotusChat-x86_64.deb")
+sha256sums=('SKIP')
+
+package() {
+  bsdtar -O -xf "$srcdir/LotusChat-x86_64.deb" data.tar.zst 2>/dev/null \
+    || bsdtar -O -xf "$srcdir/LotusChat-x86_64.deb" data.tar.xz 2>/dev/null \
+    || bsdtar -O -xf "$srcdir/LotusChat-x86_64.deb" data.tar.gz \
+    | bsdtar -C "$pkgdir" -xf -
+}