feat: [US-018] - Canvas pan and zoom controls

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/editor/[projectId]/FlowchartEditor.tsx

@@ -4,6 +4,7 @@ import { useCallback } from 'react'
 import ReactFlow, {
   Background,
   BackgroundVariant,
+  Controls,
   useNodesState,
   useEdgesState,
   addEdge,
@@ -67,6 +68,7 @@ export default function FlowchartEditor({
         fitView
       >
         <Background variant={BackgroundVariant.Dots} gap={16} size={1} />
+        <Controls position="bottom-right" />
       </ReactFlow>
     </div>
   )