implemented internal file sharing and saving, added server-side file references, refactored share pickers for unified flows, and updated UI branding labels

lib/share_intent/remote_file_ref.dart

@@ -0,0 +1,20 @@
+import '../api/marianumcloud/talk/chat/get_chat_response.dart';
+import '../api/marianumcloud/webdav/queries/list_files/cacheable_file.dart';
+
+/// References a file that already lives on the Nextcloud server. Used by the
+/// in-app share/save flows that operate on remote paths instead of local
+/// cache files (no upload needed).
+class RemoteFileRef {
+  final String path;
+  final String name;
+
+  const RemoteFileRef({required this.path, required this.name});
+
+  /// Caller must verify `file.path != null` first — Talk message parameters
+  /// without a path (system events, mentions, polls) are not file refs.
+  factory RemoteFileRef.fromTalk(RichObjectString file) =>
+      RemoteFileRef(path: file.path!, name: file.name);
+
+  factory RemoteFileRef.fromCacheable(CacheableFile file) =>
+      RemoteFileRef(path: file.path, name: file.name);
+}