Close the gaps against the design: critical tile, filters, policy card, action placement

Four things the mockup showed and the first cut did not have.

FLEET
  - A 'with critical events' tile, from the stability route that existed and
    nothing called. Clicking it narrows to rows whose health is not ok - client
    side, because the health rollup is assembled per row after pagination, so
    it narrows the PAGE rather than the fleet. Worth knowing before the count
    is read as fleet-wide.
  - PC type and Location filters. The list route already accepted both; there
    was simply no control. Both use listAll, not list: perpage is clamped to
    100 server-side, so a long location list would lose its tail in a dropdown
    while still looking complete.
  - The machine cell shows number and name together where they differ, so
    '3105 Okuma' rather than a bare number.

PC DETAIL
  - A Policy card, contributed by the computers plugin. It matters because the
    effective policy already narrows the command allowlist, so without it the
    reason a control is unavailable on a given PC is written down nowhere.
    Provenance, so it sits in the reference column, not the triage band.

    Its route returns an EMPTY field list rather than the feature-gate 404 when
    policies are disabled. The panel contract hides a card with no fields, so
    it disappears on sites that have not enabled the feature - a 404 would have
    rendered a broken card on most of them. That was flagged in the design
    review as a snag; this is the fix.

  - Refresh Status and Restart Computer moved out of the page header into the
    triage band. They act on the MACHINE; Print Label, Edit, Retire and Back to
    List act on the RECORD, and mixing them put the most consequential button
    on the page - it halts whatever machine the PC serves - between printing a
    sticker and editing a form. In the band it sits beside the liveness chip
    and the machine it would take down.

    The band takes them through an 'actions' slot rather than knowing about PC
    verbs, so a measuring tool can put 'Calibrate' there without the core
    component learning anything about computers.

Full suite 2124 passed, vitest 260 passed, gates green, spec at 333 paths.

frontend/src/api/index.js

@@ -159,6 +159,9 @@ export const endpointsApi = {
   dashboardSummary() {
     return api.get('/endpoints/dashboard/summary')
   },
+  dashboardStability() {
+    return api.get('/endpoints/dashboard/stability')
+  },
   list(params = {}) {
     return api.get('/endpoints', { params })
   },