fix(ci): install librsvg2-common so build-export can validate the SVG icon

flatpak build-export checks exported icons via gdk-pixbuf; without the
librsvg loader the bare ubuntu container rejects the app icon with
'Format not recognized'.

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

.gitea/workflows/build-flatpak.yml

@@ -42,6 +42,7 @@ jobs:
           apt-get update
           DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
             ca-certificates curl git jq flatpak wl-clipboard \
+            librsvg2-common `# gdk-pixbuf SVG loader — build-export icon validation` \
             build-essential pkg-config \
             libglib2.0-dev libdbus-1-dev libxkbcommon-dev \
             libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev \