chore: mark US-026 as complete and update progress log

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

prd.json

@@ -465,7 +465,7 @@
         "Verify in browser using dev-browser skill"
       ],
       "priority": 26,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {

progress.txt

@@ -387,3 +387,13 @@
   - Type assertion needed for select value: `e.target.value as 'set' | 'add' | 'subtract'`
   - Use `??` (nullish coalescing) for number defaults instead of `||` to allow 0 values: `data.value ?? 0`
 ---
+
+## 2026-01-21 - US-026
+- What was implemented: Add variable node from toolbar functionality
+- Files changed:
+  - src/app/editor/[projectId]/FlowchartEditor.tsx - implemented handleAddVariable to create new variable nodes at viewport center
+- **Learnings for future iterations:**
+  - handleAddVariable follows same pattern as handleAddDialogue and handleAddChoice: get viewport center, create node with nanoid, add to state
+  - Variable nodes initialized with { variableName: '', operation: 'set', value: 0 }
+  - All add node handlers share the same pattern and use the getViewportCenter helper
+---