AI chat panel with Edit/Chat modes + verify dropdown

- AI chat panel at bottom of editor (all 3 modes)
- Edit mode: AI modifies document directly (no explanations)
- Chat mode: AI answers questions about the document
- Verify dropdown: Spec Review, Grammar & Spelling, Summary
- Enter sends, Shift+Enter for newline
- /api/ai/chat endpoint with edit/chat system prompts
- Grammar and spec verify actions added to /api/ai/generate

internal/api/router.go

@@ -65,6 +65,7 @@ func NewRouter(db *sql.DB, dataDir, secret string) http.Handler {
 	// AI
 	mux.HandleFunc("POST /api/ai/verify", s.requireAuth(s.handleAIVerify))
 	mux.HandleFunc("POST /api/ai/generate", s.requireAuth(s.handleAIGenerate))
+	mux.HandleFunc("POST /api/ai/chat", s.requireAuth(s.handleAIChat))
 
 	// Build jobs
 	mux.HandleFunc("POST /api/build/submit", s.requireAuth(s.handleBuildSubmit))