fix: system health is a Settings tab, not a page only the dot reached

The surface shipped at /system with no nav entry, reachable only by
clicking the health dot beside the brand — a target you have to already
suspect something is wrong to go looking for. Operator-flagged: it needs
a path someone can walk to.

Settings is where you go to ask the instance about itself, so the view
becomes a tab there, beside Activity — Activity answers "what is the
queue doing", System answers "is anything left to do it".

- SystemView.vue moves to components/settings/SystemHealthTab.vue; the
  content is unchanged apart from shedding its own container and h1.
- SettingsView adopts useTabQuery (the composable Browse and
  Subscriptions already use) so a tab can be linked TO. The health dot
  now points at ?tab=system, and /system redirects there so the previous
  build's link and any bookmark still land.
- The tab drops its own 10s poll. v-window keeps a visited item mounted
  rather than destroyed, so that timer would have gone on firing behind
  Maintenance — and TopNav already polls the same store every 15s for
  the dot.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TTjbZZ6JirCMSaJzQV1RhA

frontend/src/views/SettingsView.vue

@@ -14,6 +14,7 @@
       style="position: sticky; top: var(--fc-nav-h, 64px); z-index: 4;"
     >
       <v-tab value="overview">Overview</v-tab>
+      <v-tab value="system">System</v-tab>
       <v-tab value="activity">Activity</v-tab>
       <v-tab value="cleanup">Cleanup</v-tab>
       <v-tab value="maintenance">Maintenance</v-tab>
@@ -42,6 +43,13 @@
         </v-alert>
       </v-window-item>
 
+      <!-- Is every part of the stack still running (milestone 365). Sits
+           beside Activity deliberately: Activity answers "what is the queue
+           doing", this answers "is anything left to do it". -->
+      <v-window-item value="system">
+        <SystemHealthTab />
+      </v-window-item>
+
       <v-window-item value="activity">
         <SystemActivityTab @open-maintenance="tab = 'maintenance'" />
       </v-window-item>
@@ -73,18 +81,24 @@
 </template>
 
 <script setup>
-import { onMounted, onUnmounted, ref, watch } from 'vue'
+import { onMounted, onUnmounted, watch } from 'vue'
 import { useSystemStore } from '../stores/system.js'
 import SystemStatsCards from '../components/settings/SystemStatsCards.vue'
 import SystemActivitySummary from '../components/settings/SystemActivitySummary.vue'
 import SystemActivityTab from '../components/settings/SystemActivityTab.vue'
+import SystemHealthTab from '../components/settings/SystemHealthTab.vue'
 import GpuActivityPanel from '../components/settings/GpuActivityPanel.vue'
 import DownloadsActivityPanel from '../components/settings/DownloadsActivityPanel.vue'
 import MaintenancePanel from '../components/settings/MaintenancePanel.vue'
 import CleanupView from './CleanupView.vue'
+import { useTabQuery } from '../composables/useTabQuery.js'
 import { useMLStore } from '../stores/ml.js'
 
-const tab = ref('overview')
+// ?tab= sync (the same composable Browse/Subscriptions use) so a tab can be
+// linked TO — the health dot beside the brand points at ?tab=system, and the
+// old /system path redirects there.
+const VALID_TABS = ['overview', 'system', 'activity', 'cleanup', 'maintenance']
+const { tab } = useTabQuery(VALID_TABS, 'overview')
 const system = useSystemStore()
 const mlStore = useMLStore()