fix(alerts/ui): bell position, content tabs hidden, filters, novice labels

Surfaced during second smoke:

1. Notification bell moved — was first child of TopBar (left of
   breadcrumb); now rendered inside the `environment` slot so it
   sits between the env selector and the user menu, matching user
   expectations.

2. Content tabs (Exchanges/Dashboard/Runtime/Deployments) hidden on
   `/alerts/*` — the operational tabs don't apply there.

3. Inbox / All alerts filters now actually filter. `AlertController.list`
   accepts only `limit` — `state`/`severity` query params are dropped
   server-side. Move `useAlerts` to fetch once per env (limit 200) and
   apply filters client-side via react-query `select`, with a stable
   queryKey so filter toggles are instant and don't re-request. True
   server-side filter needs a backend change (follow-up).

4. Novice-friendly labels:
   - Inbox subtitle: "99 firing · 100 total" → "99 need attention ·
     100 total in inbox"
   - All alerts filter: Open/Firing/Acked/All →
     "Currently open"/"Firing now"/"Acknowledged"/"All states"
   - All alerts subtitle: "N shown" → "N matching your filter"
   - History subtitle: "N resolved" → "N resolved alert(s) in range"
   - Rules subtitle: "N total" → "N rule(s) configured"
   - Silences subtitle: "N active" → "N active silence(s)" or
     "Nothing silenced right now"
   - Column headers: "State" → "Status", rules "Kind" → "Type",
     rules "Targets" → "Notifies"
   - Buttons: "Ack" → "Acknowledge", silence "End" → "End early"

Updated alerts.test.tsx and e2e selector to match new behavior/labels.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/pages/Alerts/SilencesPage.tsx

@@ -84,7 +84,7 @@ export default function SilencesPage() {
       key: 'actions', header: '', width: '90px',
       render: (_, s) => (
         <Button variant="ghost" size="sm" onClick={() => setPendingEnd(s)}>
-          End
+          End early
         </Button>
       ),
     },
@@ -95,7 +95,11 @@ export default function SilencesPage() {
       <header className={css.pageHeader}>
         <div className={css.pageTitleGroup}>
           <h2 className={css.pageTitle}>Alert silences</h2>
-          <span className={css.pageSubtitle}>{rows.length} active</span>
+          <span className={css.pageSubtitle}>
+            {rows.length === 0
+              ? 'Nothing silenced right now'
+              : `${rows.length} active silence${rows.length === 1 ? '' : 's'}`}
+          </span>
         </div>
       </header>