[mob] Add method to join public collection

mobile/lib/services/collections_service.dart

@@ -1139,6 +1139,42 @@ class CollectionsService {
     }
   }
 
+  Future<void> joinPublicCollection(
+    BuildContext context,
+    int collectionID,
+  ) async {
+    try {
+      final authToken = await getSharedPublicAlbumToken(collectionID);
+      final jwtToken = await getSharedPublicAlbumTokenJWT(collectionID);
+      final key = await getSharedPublicAlbumKey(collectionID);
+      if (key.isEmpty) {
+        throw Exception("Collection key not found");
+      }
+      final encryptedKey = CryptoUtil.sealSync(
+        getCollectionKey(collectionID),
+        CryptoUtil.base642bin(
+          Configuration.instance.getKeyAttributes()!.publicKey,
+        ),
+      );
+      await _enteDio.post(
+        "/collections/join-link",
+        data: {
+          "collectionID": collectionID,
+          "encryptedKey": CryptoUtil.bin2base64(encryptedKey),
+        },
+        options: Options(
+          headers: {
+            "X-Auth-Access-Token": authToken,
+            "X-Auth-JWT-Token": jwtToken,
+          },
+        ),
+      );
+    } catch (e) {
+      _logger.warning("Failed to join public collection $e");
+      await showGenericErrorDialog(context: context, error: e);
+    }
+  }
+
   Future<String> getSharedPublicAlbumKey(int collectionID) async {
     if (_cachedPublicAlbumKey.containsKey(collectionID)) {
       return _cachedPublicAlbumKey[collectionID]!;