fix(flatpak): pass stable branch to build-bundle (matches --default-branch)

The CI added --default-branch=stable, so the repo ref is
app/io.unom.Punktfunk/x86_64/stable. build-bundle defaults to `master` when no
branch is given → "Refspec app/io.unom.Punktfunk/x86_64/master not found". Pass
`stable` explicitly in both flatpak.yml and the local build-flatpak.sh.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/flatpak.yml

@@ -118,7 +118,9 @@ jobs:
 
       - name: Export single-file bundle
         run: |
-          flatpak build-bundle "$PWD/repo" "$BUNDLE" "$APP_ID"
+          # Branch must be passed explicitly now that the repo ref is `stable` (--default-branch
+          # above); build-bundle otherwise defaults to `master` and errors "Refspec … not found".
+          flatpak build-bundle "$PWD/repo" "$BUNDLE" "$APP_ID" stable
           ls -lh "$BUNDLE"
 
       - name: Publish to the Gitea generic registry