implemented native share intent support for android and ios with chat and folder pickers

lib/api/marianumcloud/talk/share_files_to_chat.dart

@@ -0,0 +1,21 @@
+import '../files_sharing/file_sharing_api.dart';
+import '../files_sharing/file_sharing_api_params.dart';
+
+/// WebDAV folder under which Talk-shared files are uploaded before being
+/// linked into a chat.
+const String talkShareFolder = 'MarianumMobile';
+
+/// Posts each already-uploaded WebDAV path as a Talk share (ShareType 10) to
+/// the given conversation token. Calls run concurrently — the server accepts
+/// parallel posts and the picker UI is blocked anyway, so we shouldn't pay
+/// O(n*RTT) latency per share.
+Future<void> shareFilesToChat({
+  required String token,
+  required List<String> remoteFilePaths,
+}) => Future.wait(
+  remoteFilePaths.map(
+    (path) => FileSharingApi().share(
+      FileSharingApiParams(shareType: 10, shareWith: token, path: path),
+    ),
+  ),
+);