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>

ui/src/lib/api.ts

@@ -9,6 +9,8 @@ import type {
   FeatureListResponse,
   Feature,
   FeatureCreate,
+  FeatureBulkCreate,
+  FeatureBulkCreateResponse,
   AgentStatusResponse,
   AgentActionResponse,
   SetupStatus,
@@ -111,6 +113,16 @@ export async function skipFeature(projectName: string, featureId: number): Promi
   })
 }
 
+export async function createFeaturesBulk(
+  projectName: string,
+  bulk: FeatureBulkCreate
+): Promise<FeatureBulkCreateResponse> {
+  return fetchJSON(`/projects/${encodeURIComponent(projectName)}/features/bulk`, {
+    method: 'POST',
+    body: JSON.stringify(bulk),
+  })
+}
+
 // ============================================================================
 // Agent API
 // ============================================================================