implemented Nextcloud file previews for unknown file types using fileId and has-preview flags, updated file models, and refined manual refresh logic.

lib/share_intent/remote_file_ref.dart

@@ -8,13 +8,33 @@ class RemoteFileRef {
   final String path;
   final String name;
 
-  const RemoteFileRef({required this.path, required this.name});
+  /// Nextcloud's instance-local file id, when known. Preferred over [path]
+  /// for hitting the preview API — the path-based variant is unreliable on
+  /// some server configurations.
+  final int? fileId;
+
+  /// Server's `nc:has-preview` flag when known. null = unknown.
+  final bool? hasPreview;
+
+  const RemoteFileRef({
+    required this.path,
+    required this.name,
+    this.fileId,
+    this.hasPreview,
+  });
 
   /// 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.fromTalk(RichObjectString file) => RemoteFileRef(
+    path: file.path!,
+    name: file.name,
+    fileId: int.tryParse(file.id),
+  );
 
-  factory RemoteFileRef.fromCacheable(CacheableFile file) =>
-      RemoteFileRef(path: file.path, name: file.name);
+  factory RemoteFileRef.fromCacheable(CacheableFile file) => RemoteFileRef(
+    path: file.path,
+    name: file.name,
+    fileId: file.fileId,
+    hasPreview: file.hasPreview,
+  );
 }