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/routers/__init__.py

@@ -12,6 +12,7 @@ from .expand_project import router as expand_project_router
 from .features import router as features_router
 from .filesystem import router as filesystem_router
 from .projects import router as projects_router
+from .scaffold import router as scaffold_router
 from .schedules import router as schedules_router
 from .settings import router as settings_router
 from .spec_creation import router as spec_creation_router
@@ -29,4 +30,5 @@ __all__ = [
     "assistant_chat_router",
     "settings_router",
     "terminal_router",
+    "scaffold_router",
 ]