feat: Implement TreeView command handlers (expand, select, persist, restore)

[NikolaRHristov]

Context

Command/TreeView.rs has 4 stub command handlers that return {"success": false, "error": "not implemented"}.

Stubs to Implement

• OnTreeViewExpansionChanged — Notify registered TreeViewProviders of expansion state changes
• OnTreeViewSelectionChanged — Forward selection events to providers
• PersistTreeView — Serialize tree state to storage for workspace persistence
• RestoreTreeView — Deserialize and restore tree state on workspace open

Expected Behavior

Each handler should route through the TreeViewProvider trait to Cocoon gRPC RPCs.

Priority

High — File explorer tree and extension tree views depend on this.