fix(flatpak): use GNOME SDK 48 with manual Node/Rust install

- Switch to GNOME SDK 48 which includes webkit2gtk-4.1
- Install Node.js 20 LTS directly from tarball (no SDK extension needed)
- Install Rust via rustup for latest stable version
- Add flatpak build directories to .gitignore

.github/workflows/release-flatpak.yml

@@ -77,9 +77,8 @@ jobs:
       - name: Install runtimes
         run: |
           flatpak --user install -y --noninteractive flathub \
-            org.freedesktop.Platform//23.08 \
-            org.freedesktop.Sdk//23.08 \
-            org.freedesktop.Sdk.Extension.node20//23.08
+            org.gnome.Platform//48 \
+            org.gnome.Sdk//48
 
       - name: Build Flatpak
         env: