Fix flaky E2E tests with proper selectors and response waits

- Use data-testid attributes to target specific day cards in availability tests
- Wait for networkidle before interacting with page elements
- Set up PUT and GET response listeners before triggering actions
- Add retry logic for availability API in booking tests
- Fix appointments test to handle multiple 'Booked' elements with .first()
- Increase parallel workers to 12 for faster test execution

frontend/e2e/booking.spec.ts

@@ -52,8 +52,8 @@ function getTomorrowDateStr(): string {
   return formatDateLocal(tomorrow);
 }
 
-// Set up availability for a date using the API
-async function setAvailability(page: Page, dateStr: string) {
+// Set up availability for a date using the API with retry logic
+async function setAvailability(page: Page, dateStr: string, maxRetries = 3) {
   const cookies = await page.context().cookies();
   const authCookie = cookies.find(c => c.name === "auth_token");
   
@@ -61,21 +61,39 @@ async function setAvailability(page: Page, dateStr: string) {
     throw new Error("No auth cookie found when trying to set availability");
   }
   
-  const response = await page.request.put(`${API_URL}/api/admin/availability`, {
-    headers: {
-      Cookie: `auth_token=${authCookie.value}`,
-      "Content-Type": "application/json",
-    },
-    data: {
-      date: dateStr,
-      slots: [{ start_time: "09:00:00", end_time: "12:00:00" }],
-    },
-  });
+  let lastError: Error | null = null;
   
-  if (!response.ok()) {
+  for (let attempt = 0; attempt < maxRetries; attempt++) {
+    if (attempt > 0) {
+      // Wait before retry
+      await page.waitForTimeout(500);
+    }
+    
+    const response = await page.request.put(`${API_URL}/api/admin/availability`, {
+      headers: {
+        Cookie: `auth_token=${authCookie.value}`,
+        "Content-Type": "application/json",
+      },
+      data: {
+        date: dateStr,
+        slots: [{ start_time: "09:00:00", end_time: "12:00:00" }],
+      },
+    });
+    
+    if (response.ok()) {
+      return; // Success
+    }
+    
     const body = await response.text();
-    throw new Error(`Failed to set availability: ${response.status()} - ${body}`);
+    lastError = new Error(`Failed to set availability: ${response.status()} - ${body}`);
+    
+    // Only retry on 500 errors
+    if (response.status() !== 500) {
+      throw lastError;
+    }
   }
+  
+  throw lastError;
 }
 
 test.describe("Booking Page - Regular User Access", () => {