desktop CI: pin upload-artifact@v3 (Forgejo rejects v4 protocol)

The build produces the .deb + .AppImage fine; upload-artifact@v4 failed with
GHESNotSupportedError (Forgejo has no v4 artifact API). v3 uses the older protocol
the instance accepts, so the bundles become downloadable from the run for
hand-testing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

.forgejo/workflows/desktop.yml

@@ -70,9 +70,11 @@ jobs:
       # Make the built .deb + .AppImage downloadable from the run (for hand-testing).
       # continue-on-error: the Forgejo artifact backend may not be configured yet; a
       # failed upload must not fail the build itself.
+      # Forgejo doesn't support the v4 artifact protocol (@actions/artifact v2+),
+      # so pin v3, which uses the older protocol the instance accepts.
       - name: Upload bundles
         continue-on-error: true
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: thoughtsync-linux
           path: |