Add Firestore rules for sharedPrayers collection to enable community sharing functionality

firestore.rules

@@ -29,9 +29,12 @@ service cloud.firestore {
       allow read, write: if request.auth != null;
     }
 
-    // Allow anyone to read public data (if needed in future)
-    match /public/{document} {
-      allow read: if true;
+    // Allow authenticated users to read/write shared prayers
+    match /sharedPrayers/{prayerId} {
+      allow read: if true; // Anyone can read shared prayers
+      allow create: if request.auth != null; // Only authenticated users can share
+      allow write: if request.auth != null && 
+        request.auth.uid == resource.data.userId; // Only the original sharer can update
     }
   }
 }