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/test/router.spec.js

@@ -40,6 +40,14 @@ describe('router', () => {
     expect(router.currentRoute.value.query.post_id).toBe('7')
   })
 
+  it('/system redirects into the Settings System tab', async () => {
+    // It shipped as a standalone page for one build; the health dot and any
+    // bookmark from it must still land on the surface, which is now a tab.
+    await router.push('/system')
+    expect(router.currentRoute.value.name).toBe('settings')
+    expect(router.currentRoute.value.query.tab).toBe('system')
+  })
+
   it('series-read is an immersive route', () => {
     const r = router.resolve('/series/5/read')
     expect(r.name).toBe('series-read')