Configurable list columns via header right-click

Native checkable menu on the column headers toggles Size, Modified,
Kind, Rights, Owner and Group (persisted, both panels). Owner/group
come from the ls-style SFTP longname remotely and cached uid/gid
lookups locally (POSIX; undefined on Windows). All columns sort.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/shared/rpc.ts

@@ -1,5 +1,6 @@
 import type {
 	AppSettings,
+	ColumnKey,
 	ConnectionConfig,
 	ConnectionsFile,
 	EntryType,
@@ -134,6 +135,8 @@ export type AppRPC = {
 				response: Result;
 			};
 			moveToTrash: { params: { path: string }; response: Result };
+			/** Native column-picker menu (right-click on the list headers). */
+			showColumnsMenu: { params: { visible: ColumnKey[] }; response: Result };
 			// editor windows
 			/** Main window only: open (or focus) a native editor window. */
 			openEditor: { params: EditorOpenContext; response: Result };