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/bun/rpc-handlers.ts

@@ -275,6 +275,12 @@ export function createAppRPC({
 						return {};
 					}),
 
+				showColumnsMenu: ({ visible }) =>
+					wrap(async () => {
+						contextMenu.showColumns(visible);
+						return {};
+					}),
+
 				// ---- editor windows ----
 				openEditor: (ctx) =>
 					wrap(async () => {