fix: Include start_date in goal edit form and API call

**Bug:** start_date was not being loaded into edit form or sent in update request

**Fixed:**
1. handleEditGoal() - Added start_date to formData when editing
2. handleSaveGoal() - Added start_date to API payload for both create and update

Now when editing a goal:
- start_date field is populated with existing value
- Changing start_date triggers backend to recalculate start_value
- Update request includes start_date

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/GoalsPage.jsx

@@ -196,6 +196,7 @@ export default function GoalsPage() {
       priority: goal.priority || 2,
       target_value: goal.target_value,
       unit: goal.unit,
+      start_date: goal.start_date || new Date().toISOString().split('T')[0],
       target_date: goal.target_date || '',
       name: goal.name || '',
       description: goal.description || '',
@@ -227,6 +228,7 @@ export default function GoalsPage() {
         priority: formData.priority,
         target_value: parseFloat(formData.target_value),
         unit: formData.unit,
+        start_date: formData.start_date || null,
         target_date: formData.target_date || null,
         name: formData.name || null,
         description: formData.description || null,