fix(server): apply lifecycle reply state optimistically (heartbeat lag)

The agent reply is authoritative for the action just taken; the fleet
DB only updates on the next heartbeat (~10s), so the immediate refetch
read a stale state and reverted the UI (Start -> still Stopped). Now
apply the reply's state/uptime directly to the instance.

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

frontend/src/stores/instances.ts

@@ -68,8 +68,10 @@ export const useInstancesStore = defineStore('instances', () => {
   }
 
   /**
-   * Send a lifecycle command to the current instance. Returns the agent's
-   * reply (which carries the new state); refetches so the list reflects it.
+   * Send a lifecycle command to the current instance and apply the agent's
+   * reply state OPTIMISTICALLY. The reply is authoritative for the action just
+   * taken; the fleet DB only catches up on the next heartbeat (~10s), so an
+   * immediate refetch would read a stale state and clobber the result.
    * Throws on failure so the view can toast.
    */
   async function lifecycle(action: LifecycleAction): Promise<Record<string, unknown>> {
@@ -78,13 +80,26 @@ export const useInstancesStore = defineStore('instances', () => {
     acting.value = action
     try {
       const res = await api.post<Record<string, unknown>>(`/instances/${id}/lifecycle`, { action })
-      await fetchInstances()
+      applyReplyState(id, res)
       return res
     } finally {
       acting.value = null
     }
   }
 
+  /** Update an instance's state/uptime from a lifecycle/status reply. */
+  function applyReplyState(id: string, res: Record<string, unknown>): void {
+    if ((res as { status?: string }).status !== 'success') return
+    const stateObj = (res as { state?: { state?: string } }).state
+    const newState = stateObj?.state
+    const inst = instances.value.find((i) => i.id === id)
+    if (inst && typeof newState === 'string') {
+      inst.state = newState
+      const up = (res as { uptime_seconds?: number }).uptime_seconds
+      inst.uptime_seconds = typeof up === 'number' ? up : newState === 'running' ? inst.uptime_seconds : 0
+    }
+  }
+
   async function rcon(command: string): Promise<Record<string, unknown>> {
     const id = currentId.value
     if (!id) throw new Error('No instance selected')