chore: mark US-027 as complete and update progress log

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

prd.json

@@ -483,7 +483,7 @@
         "Verify in browser using dev-browser skill"
       ],
       "priority": 27,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {

progress.txt

@@ -397,3 +397,15 @@
   - Variable nodes initialized with { variableName: '', operation: 'set', value: 0 }
   - All add node handlers share the same pattern and use the getViewportCenter helper
 ---
+
+## 2026-01-21 - US-027
+- What was implemented: Connect nodes with edges including arrow markers and smooth styling
+- Files changed:
+  - src/app/editor/[projectId]/FlowchartEditor.tsx - added MarkerType import, updated onConnect to create edges with smoothstep type and arrow markers, updated toReactFlowEdges to apply same styling to loaded edges
+- **Learnings for future iterations:**
+  - Use `type: 'smoothstep'` for cleaner edge curves instead of default bezier
+  - Use `markerEnd: { type: MarkerType.ArrowClosed }` to add directional arrows to edges
+  - Connection type has nullable source/target, but Edge requires non-null strings - guard with early return
+  - Apply consistent edge styling in both onConnect (new edges) and toReactFlowEdges (loaded edges)
+  - Generate unique edge IDs with nanoid in onConnect callback
+---