Add global web nav with project groups and tree sidebar.

Restructure the UI into top-level tabs and a scrollable project tree, and persist optional project groups so demos and settings can organize many projects.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/project/project.go

@@ -2,6 +2,7 @@ package project
 
 // Project is a lightweight workspace identified by a directory name.
 type Project struct {
-	ID   string `json:"id"`
-	Name string `json:"name"`
+	ID      string `json:"id"`
+	Name    string `json:"name"`
+	GroupID string `json:"groupId,omitempty"`
 }