chore: mark US-018 as complete and update progress log

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

prd.json

@@ -318,7 +318,7 @@
         "Verify in browser using dev-browser skill"
       ],
       "priority": 18,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {

progress.txt

@@ -282,3 +282,13 @@
   - Background component accepts variant (Dots, Lines, Cross) and gap/size props
   - Loading page (loading.tsx) provides automatic loading UI for async server components
 ---
+
+## 2026-01-21 - US-018
+- What was implemented: Canvas pan and zoom controls
+- Files changed:
+  - src/app/editor/[projectId]/FlowchartEditor.tsx - added Controls import and component
+- **Learnings for future iterations:**
+  - React Flow Controls component provides zoom +/-, fitView, and lock buttons out of the box
+  - Use position="bottom-right" prop to position controls in bottom-right corner
+  - Pan (click-and-drag) and zoom (mouse wheel) are React Flow defaults, no extra config needed
+---