feat: add scaffold router and project template selection step

Add a new scaffold system that lets users choose a project template
(blank or agentic starter) during project creation. This inserts a
template selection step between folder selection and spec method choice.

Backend:
- New server/routers/scaffold.py with SSE streaming endpoint for
  running hardcoded scaffold commands (npx create-agentic-app)
- Path validation, security checks, and cross-platform npx resolution
- Registered scaffold_router in server/main.py and routers/__init__.py

Frontend (NewProjectModal.tsx):
- New "template" step with Blank Project and Agentic Starter cards
- Real-time scaffold output streaming with auto-scroll log viewer
- Success, error, and retry states with proper back-navigation
- Updated step flow: name → folder → template → method → chat/complete

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/main.py

@@ -36,6 +36,7 @@ from .routers import (
     features_router,
     filesystem_router,
     projects_router,
+    scaffold_router,
     schedules_router,
     settings_router,
     spec_creation_router,
@@ -169,6 +170,7 @@ app.include_router(filesystem_router)
 app.include_router(assistant_chat_router)
 app.include_router(settings_router)
 app.include_router(terminal_router)
+app.include_router(scaffold_router)
 
 
 # ============================================================================