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/schemas.py

@@ -96,6 +96,18 @@ class FeatureListResponse(BaseModel):
     done: list[FeatureResponse]
 
 
+class FeatureBulkCreate(BaseModel):
+    """Request schema for bulk creating features."""
+    features: list[FeatureCreate]
+    starting_priority: int | None = None  # If None, appends after max priority
+
+
+class FeatureBulkCreateResponse(BaseModel):
+    """Response for bulk feature creation."""
+    created: int
+    features: list[FeatureResponse]
+
+
 # ============================================================================
 # Agent Schemas
 # ============================================================================