feat: Add "Expand Project" for bulk AI-powered feature creation

Adds the ability to add multiple features to an existing project through
a natural language conversation with Claude, similar to how initial spec
creation works.

Features:
- New "Expand" button in header (keyboard shortcut: E)
- Full-screen chat interface for describing new features
- Claude reads existing app_spec.txt for context
- Features created directly in database after user approval
- Bulk feature creation endpoint for batch operations

New files:
- .claude/commands/expand-project.md - Claude skill for expansion
- server/services/expand_chat_session.py - Chat session service
- server/routers/expand_project.py - WebSocket endpoint
- ui/src/components/ExpandProjectChat.tsx - Chat UI
- ui/src/components/ExpandProjectModal.tsx - Modal wrapper
- ui/src/hooks/useExpandChat.ts - WebSocket hook

Modified:
- Added POST /bulk endpoint to features router
- Added FeatureBulkCreate schemas
- Integrated Expand button and modal in App.tsx

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

server/routers/__init__.py

@@ -7,6 +7,7 @@ FastAPI routers for different API endpoints.
 
 from .agent import router as agent_router
 from .assistant_chat import router as assistant_chat_router
+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
@@ -17,6 +18,7 @@ __all__ = [
     "features_router",
     "agent_router",
     "spec_creation_router",
+    "expand_project_router",
     "filesystem_router",
     "assistant_chat_router",
 ]