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

desktopApp/build.gradle.kts

@@ -133,7 +133,10 @@ compose.desktop {
 }
 
 vlcSetup {
-    vlcVersion.set("3.0.21")
+    // Pinned to 3.0.20 because the Linux VLC plugins on Maven Central
+    // (ir.mahozad:vlc-plugins-linux) have not been republished for 3.0.21 — the
+    // latest there is 3.0.20-2. Using 3.0.21 makes vlcDownload 404 on Linux CI.
+    vlcVersion.set("3.0.20")
     shouldCompressVlcFiles.set(true)
     shouldIncludeAllVlcFiles.set(true)
     pathToCopyVlcLinuxFilesTo.set(file("src/jvmMain/appResources/linux/vlc"))