feat: pre-populate heats form with existing data when editing

- Add existingHeats prop to HeatsBanner component
- Load and format existing heats into form fields
- Pass myHeats to HeatsBanner in edit modal
- Users can now edit their heats instead of starting from scratch

frontend/src/pages/EventChatPage.jsx

@@ -717,6 +717,7 @@ const EventChatPage = () => {
                   slug={slug}
                   onSave={handleHeatsSave}
                   onDismiss={() => setShowHeatsModal(false)}
+                  existingHeats={myHeats}
                 />
               </div>
             </div>