feat(server): instance-centric controls — real per-instance state + lifecycle

The Server page now manages the selected GAME INSTANCE, not the legacy
host connection. New instances store flattens the fleet and drives the
command bridge. New 'Game instance' panel: real state badge
(running/stopped/crashed/configured), uptime, host, and an instance
selector when >1. Start/Stop/Restart/Refresh wired to POST
/api/instances/:id/lifecycle — gated on the actual instance state (not
host connectivity), with telemetry-only instances flagged. Works across
all four games (state + lifecycle are game-agnostic).

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

frontend/src/stores/instances.ts

@@ -0,0 +1,106 @@
+import { defineStore } from 'pinia'
+import { ref, computed } from 'vue'
+import { useApi } from '@/composables/useApi'
+import type { FleetData, FleetInstance } from '@/stores/fleet'
+
+/** A game instance enriched with its host context, flattened from the fleet. */
+export interface ManagedInstance extends FleetInstance {
+  host_id: string
+  host_hostname: string
+  host_status: string
+}
+
+type LifecycleAction = 'start' | 'stop' | 'restart' | 'status' | 'steam_update'
+
+/**
+ * Instance management — the Server page operates on a selected game instance
+ * (not the legacy single-server connection). Reads the fleet to enumerate
+ * instances and drives the per-instance command bridge
+ * (POST /api/instances/:id/lifecycle | /rcon).
+ */
+export const useInstancesStore = defineStore('instances', () => {
+  const instances = ref<ManagedInstance[]>([])
+  const currentId = ref<string | null>(null)
+  const loading = ref(false)
+  const acting = ref<LifecycleAction | null>(null)
+  const error = ref<string | null>(null)
+
+  const api = useApi()
+
+  const current = computed<ManagedInstance | null>(
+    () => instances.value.find((i) => i.id === currentId.value) ?? null,
+  )
+
+  /** Fetch the fleet and flatten its instances. Optionally prefer a game. */
+  async function fetchInstances(preferGame?: string): Promise<void> {
+    loading.value = true
+    error.value = null
+    try {
+      const data = await api.get<FleetData>('/fleet')
+      const flat: ManagedInstance[] = []
+      for (const host of data.hosts) {
+        for (const inst of host.instances) {
+          flat.push({
+            ...inst,
+            host_id: host.id,
+            host_hostname: host.hostname,
+            host_status: host.status,
+          })
+        }
+      }
+      instances.value = flat
+      // Keep the current selection if it still exists; else prefer the active
+      // game, else the first instance.
+      if (!flat.some((i) => i.id === currentId.value)) {
+        const preferred = preferGame ? flat.find((i) => i.game === preferGame) : undefined
+        currentId.value = (preferred ?? flat[0])?.id ?? null
+      }
+    } catch (e) {
+      console.error('Failed to fetch instances:', e)
+      error.value = e instanceof Error ? e.message : 'Failed to load instances'
+    } finally {
+      loading.value = false
+    }
+  }
+
+  function select(id: string): void {
+    currentId.value = id
+  }
+
+  /**
+   * Send a lifecycle command to the current instance. Returns the agent's
+   * reply (which carries the new state); refetches so the list reflects it.
+   * Throws on failure so the view can toast.
+   */
+  async function lifecycle(action: LifecycleAction): Promise<Record<string, unknown>> {
+    const id = currentId.value
+    if (!id) throw new Error('No instance selected')
+    acting.value = action
+    try {
+      const res = await api.post<Record<string, unknown>>(`/instances/${id}/lifecycle`, { action })
+      await fetchInstances()
+      return res
+    } finally {
+      acting.value = null
+    }
+  }
+
+  async function rcon(command: string): Promise<Record<string, unknown>> {
+    const id = currentId.value
+    if (!id) throw new Error('No instance selected')
+    return api.post<Record<string, unknown>>(`/instances/${id}/rcon`, { command })
+  }
+
+  return {
+    instances,
+    currentId,
+    current,
+    loading,
+    acting,
+    error,
+    fetchInstances,
+    select,
+    lifecycle,
+    rcon,
+  }
+})