fix(desktop): pin vlcVersion to 3.0.20 so Linux vlcDownload finds an artifact

The Linux build path of the vlc-setup plugin pulls vlc-plugins-linux from
Maven Central (ir.mahozad:vlc-plugins-linux), which only ships 3.0.20 and
3.0.20-2 — there is no 3.0.21 artifact yet. With vlcVersion set to 3.0.21,
both the CI pre-fetch step and the in-Gradle vlcDownload task hit a 404.

Match VLC_VERSION in build.yml and document the lag so future bumps wait
for the Maven artifact to catch up.

https://claude.ai/code/session_01GrZLMi3sdp6frwREmQ9cUi

.github/workflows/build.yml

@@ -111,11 +111,14 @@ jobs:
       # populating the cache for the next run.
       #
       # Versions are pinned to match desktopApp/build.gradle.kts (vlcVersion
-      # = 3.0.21) and the vlc-setup extension default (upxVersion = 4.2.4).
+      # = 3.0.20) and the vlc-setup extension default (upxVersion = 4.2.4).
+      # NOTE: vlcVersion lags behind upstream VLC because the Linux plugins on
+      # Maven Central (ir.mahozad:vlc-plugins-linux) are only published for
+      # 3.0.20 / 3.0.20-2. Bump only after the Maven artifact is republished.
       # macOS does not download UPX — UPX cannot compress .dylib files.
       - name: Pre-fetch VLC + UPX archives
         env:
-          VLC_VERSION: "3.0.21"
+          VLC_VERSION: "3.0.20"
           UPX_VERSION: "4.2.4"
         run: |
           set -euo pipefail