feat(v2): add frontend remote machine integration

remote-bridge.ts adapter for machine management IPC. machines.svelte.ts
store for remote machine state. Layout store extended with
remoteMachineId on Pane interface. agent-bridge.ts and pty-bridge.ts
route to remote commands when remoteMachineId is set. SettingsDialog
gains Remote Machines section. Sidebar auto-groups remote panes by
machine label.

v2/src/lib/stores/layout.svelte.ts

@@ -23,6 +23,7 @@ export interface Pane {
   args?: string[];
   group?: string;
   focused: boolean;
+  remoteMachineId?: string;
 }
 
 let panes = $state<Pane[]>([]);