fix: address PR review feedback from CodeRabbit

- Upgrade softprops/action-gh-release@v1 to @v2 (Node 16 EOL)
- Fail-fast on checksum fetch failure instead of extracting unverified archives
- Abort packaging if no NVIDIA files found (prevents empty cuda-libs archive)
- Fix nvidia/ path detection bug (list membership vs substring check)
- Fix justfile Copy-Item nesting (copy contents, not the directory itself)

justfile

@@ -210,7 +210,8 @@ build-server-cuda: _ensure-venv
     if ($LASTEXITCODE -ne 0) { throw "build_binary.py --cuda failed with exit code $LASTEXITCODE" }; \
     $dest = "$env:APPDATA/com.voicebox.app/backends/cuda"; \
     if (Test-Path $dest) { Remove-Item -Recurse -Force $dest }; \
-    Copy-Item "backend/dist/voicebox-server-cuda" $dest -Recurse -Force; \
+    New-Item -ItemType Directory -Path $dest -Force | Out-Null; \
+    Copy-Item "backend/dist/voicebox-server-cuda/*" $dest -Recurse -Force; \
     Write-Host "Copied CUDA backend to $dest"
 
 # Build everything locally: CPU server + CUDA server + installable Tauri app