Bring in mcp-skills-plugins branch: marketplace, plugins, skills, OpenCredits, and image previews

Major release adding:
- MCP marketplace with curated registry and search across multiple registries
- Plugins and skills marketplace integration
- OpenCredits payment integration with model selection and checkout flow
- Image preview before sending (paste, file picker)
- Self-hosted Umami analytics with custom events
- Support feedback modal with Discord webhook
- Local OpenAI to Anthropic router for model routing
- Inline stop button replacing send during processing
- Improved install flow requiring Node.js 18+
- WSL env var passthrough fixes
- Better error handling and Windows compatibility

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude/settings.json

@@ -0,0 +1,5 @@
+{
+  "enabledPlugins": {
+    "frontend-design@claude-plugins-official": true
+  }
+}