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/components/ExpandProjectModal.tsx

@@ -0,0 +1,41 @@
+/**
+ * Expand Project Modal
+ *
+ * Full-screen modal wrapper for the ExpandProjectChat component.
+ * Allows users to add multiple features to an existing project via AI.
+ */
+
+import { ExpandProjectChat } from './ExpandProjectChat'
+
+interface ExpandProjectModalProps {
+  isOpen: boolean
+  projectName: string
+  onClose: () => void
+  onFeaturesAdded: () => void  // Called to refresh feature list
+}
+
+export function ExpandProjectModal({
+  isOpen,
+  projectName,
+  onClose,
+  onFeaturesAdded,
+}: ExpandProjectModalProps) {
+  if (!isOpen) return null
+
+  const handleComplete = (featuresAdded: number) => {
+    if (featuresAdded > 0) {
+      onFeaturesAdded()
+    }
+    onClose()
+  }
+
+  return (
+    <div className="fixed inset-0 z-50 bg-[var(--color-neo-bg)]">
+      <ExpandProjectChat
+        projectName={projectName}
+        onComplete={handleComplete}
+        onCancel={onClose}
+      />
+    </div>
+  )
+}