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/internal_share_actions.dart

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+import 'package:flutter/material.dart';
+import 'package:nextcloud/nextcloud.dart';
+
+import '../api/marianumcloud/webdav/webdav_api.dart';
+import '../widget/confirm_dialog.dart';
+import 'remote_file_ref.dart';
+
+/// Server-side WebDAV copy of [source] into [targetFolderPath]. On a 412
+/// conflict the user is asked whether to overwrite; on confirmation the call
+/// is retried with `overwrite: true`. Returns true when the file ended up at
+/// the target, false when the user cancelled.
+Future<bool> copyRemoteFileTo({
+  required BuildContext context,
+  required RemoteFileRef source,
+  required String targetFolderPath,
+}) async {
+  final webdav = await WebdavApi.webdav;
+  final dst = targetFolderPath.isEmpty
+      ? source.name
+      : '${targetFolderPath.replaceAll(RegExp(r'/+$'), '')}/${source.name}';
+  final src = PathUri.parse(source.path);
+  final dstUri = PathUri.parse(dst);
+
+  try {
+    await webdav.copy(src, dstUri);
+    return true;
+  } on DynamiteApiException catch (e) {
+    if (e.statusCode != 412) rethrow;
+    if (!context.mounted) return false;
+    final overwrite = await showDialog<bool>(
+      context: context,
+      builder: (ctx) => ConfirmDialog(
+        title: 'Datei existiert bereits',
+        content:
+            '"${source.name}" existiert in /$targetFolderPath. Überschreiben?',
+        confirmButton: 'Überschreiben',
+        cancelButton: 'Abbrechen',
+        onConfirm: () => Navigator.of(ctx).pop(true),
+      ),
+    );
+    if (overwrite != true) return false;
+    await webdav.copy(src, dstUri, overwrite: true);
+    return true;
+  }
+}

lib/share_intent/remote_file_ref.dart

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+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);
+}