feat(v2): add sidecar crash detection, restart UI, and auto-scroll lock

Phase 3 polish: dispatcher listens for sidecar-exited events and marks
running sessions as error. AgentPane shows "Restart Sidecar" button on
error. Auto-scroll disables when user scrolls >50px from bottom with
"Scroll to bottom" button. Added agent_restart Tauri command.

v2/src/lib/adapters/agent-bridge.ts

@@ -25,6 +25,10 @@ export async function isAgentReady(): Promise<boolean> {
   return invoke<boolean>('agent_ready');
 }
 
+export async function restartAgent(): Promise<void> {
+  return invoke('agent_restart');
+}
+
 export interface SidecarMessage {
   type: string;
   sessionId?: string;