Fix CI build failures on self-hosted runners

- Move bufferutil to optionalDependencies (not required for core functionality)
- Add --no-optional flag to npm ci in workflow to skip native dependencies
- This avoids needing C++ build tools on CI runners

The bufferutil package is a performance optimization for WebSocket but not required.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.forgejo/workflows/build.yml

@@ -19,7 +19,7 @@ jobs:
         run: rm -rf .next
 
       - name: Install dependencies
-        run: npm ci
+        run: npm ci --no-optional
 
       - name: Run type check
         run: npm run type-check