fix: serialize date objects to ISO format for JSON

- Added serialize_dates() helper to convert date objects to strings
- Applied to list_goals and get_goals_grouped endpoints
- Fixes issue where start_date was saved but not visible in frontend
- Python datetime.date objects need explicit .isoformat() conversion

Root cause: FastAPI doesn't auto-serialize all date types consistently

backend/routers/goals.py

@@ -25,6 +25,19 @@ from goal_utils import get_current_value_for_goal
 
 router = APIRouter(prefix="/api/goals", tags=["goals"])
 
+
+def serialize_dates(obj):
+    """Convert date/datetime objects to ISO format strings for JSON serialization."""
+    if obj is None:
+        return None
+    if isinstance(obj, dict):
+        return {k: serialize_dates(v) for k, v in obj.items()}
+    if isinstance(obj, list):
+        return [serialize_dates(item) for item in obj]
+    if isinstance(obj, (date,)):
+        return obj.isoformat()
+    return obj
+
 # ============================================================================
 # Pydantic Models
 # ============================================================================
@@ -362,6 +375,9 @@ def list_goals(session: dict = Depends(require_auth)):
                     print(f"[ERROR] Failed to update progress for goal {goal.get('id')}: {e}")
                     # Continue with other goals even if one fails
 
+            # Serialize date objects to ISO format strings
+            goals = serialize_dates(goals)
+
             return goals
 
     except Exception as e:
@@ -700,6 +716,9 @@ def get_goals_grouped(session: dict = Depends(require_auth)):
             goal_dict['focus_contributions'] = focus_map.get(goal['id'], [])
             grouped[cat].append(goal_dict)
 
+        # Serialize date objects to ISO format strings
+        grouped = serialize_dates(grouped)
+
         return grouped
 
 # ============================================================================