Add project creation from team dashboard and update seed data

- Add project creation modal to TeamDashboardPage with team_id assignment
- Update createProject API function to accept optional team_id
- Update seed data to create a "Demo Team" and assign all projects to it
- Admin user is added as team owner when present

frontend/src/api.ts

@@ -166,7 +166,7 @@ export async function listProjectsSimple(params: ListParams = {}): Promise<Proje
   return data.items;
 }
 
-export async function createProject(data: { name: string; description?: string; is_public?: boolean }): Promise<Project> {
+export async function createProject(data: { name: string; description?: string; is_public?: boolean; team_id?: string }): Promise<Project> {
   const response = await fetch(`${API_BASE}/projects`, {
     method: 'POST',
     headers: { 'Content-Type': 'application/json' },