Add Plugin marketplace browser UI

[PureWeen]

Summary

The CLI has a plugin ecosystem with marketplace support — install, list, and discover plugins from curated marketplaces (copilot-plugins, awesome-copilot). PolyPilot has no UI for this despite having 30+ plugins installed.

CLI Commands

• copilot plugin install <name> — install a plugin
• copilot plugin list — list installed plugins
• copilot plugin marketplace — browse available plugins
• /plugin — interactive plugin management

SDK API

• ServerRpc.Plugins.ListAsync() — list installed plugins

Proposed Implementation

1. Plugin browser page — grid/list of available plugins from marketplace
2. Installed plugins panel — show what's installed with version info
3. Install/Uninstall buttons with progress feedback
4. Plugin details — description, author, tool count, usage stats
5. Search/filter by category, popularity, compatibility

Priority: 🟡 MEDIUM