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/components/TopNav.vue

@@ -6,7 +6,8 @@
         <span class="fc-brand__text">FabledCurator</span>
       </RouterLink>
       <RouterLink
-        :to="{ name: 'system' }" class="fc-health" :title="health.label"
+        :to="{ name: 'settings', query: { tab: 'system' } }"
+        class="fc-health" :title="health.label"
         :aria-label="`System health: ${health.label}`"
       >
         <v-icon size="x-small" :color="health.color">{{ health.icon }}</v-icon>
@@ -282,9 +283,9 @@ onUnmounted(() => { if (healthTimer) clearInterval(healthTimer) })
   display: flex;
   align-items: center;
   flex-shrink: 0;
-  /* A RouterLink since milestone 365 — it is the path to /system, not just an
-     indicator. Reset the anchor so turning a span into a link changed nothing
-     about how the nav reads. */
+  /* A RouterLink since milestone 365 — it is the path to the Settings System
+     tab, not just an indicator. Reset the anchor so turning a span into a
+     link changed nothing about how the nav reads. */
   text-decoration: none;
   color: inherit;
   border-radius: 50%;