Key-file picker in the connection manager

Choose... button beside the private-key field opens the native file dialog
starting in ~/.ssh (Utils.openFileDialog via a pickKeyFile RPC).

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

src/shared/rpc.ts

@@ -91,6 +91,10 @@ export type AppRPC = {
 				params: Record<string, never>;
 				response: Result<{ path: string | null }>;
 			};
+			pickKeyFile: {
+				params: Record<string, never>;
+				response: Result<{ path: string | null }>;
+			};
 			getEditLeftovers: {
 				params: Record<string, never>;
 				response: Result<{ names: string[] }>;