fix(a11y): clear axe-core violations on /admin and key sub-routes

Re-enable the /admin axe-core spec that phase-3 parked while phase-4b
restructured the admin into per-section dynamic routes. The fixme is
gone, /admin/system-status and /admin/users are added as representative
sub-route audits, and the violations the original audit surfaced are
fixed:

- `src/components/admin/_shared.tsx`: drop the duplicate
  `<PasswordInput>` whose toggle button had no accessible name.
  Re-export the canonical `<PasswordInput>` from
  `src/components/settings/password-input.tsx` (added in phase-3) which
  already carries the `aria-label="Show password" / "Hide password"`
  pair. Affects `/admin/integrations` (bug-report token, web-push VAPID
  private key) and `/admin/users` (password-reset dialog).
- `src/components/admin/user-management-section.tsx`: icon-only edit /
  reset-password / force-logout buttons now carry `aria-label` (axe
  `button-name` is `serious`); the leading icon is `aria-hidden` so the
  accessible name is just the action verb.
- `src/components/admin/status-card-grid.tsx`: drop the placeholder
  `{ label: "—", value: "" }` tuple from the audit-log card. An empty
  `<dd>` next to a "—" `<dt>` is not navigable and the `<dl>` was the
  most likely source of an axe violation on `/admin`. The 3-column grid
  still works with two metrics; the empty cell at the end is unrendered
  because there is no `<div>` for it.
- `src/components/admin/__tests__/status-card-grid.test.tsx`: rework
  the 3-metric assertion to allow 1..3 metrics with non-empty,
  non-em-dash labels so the audit-log change doesn't regress.

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

src/components/admin/__tests__/status-card-grid.test.tsx

@@ -141,10 +141,19 @@ describe("buildCards()", () => {
     }
   });
 
-  it("includes 3 metric tuples per card", () => {
+  it("includes between 1 and 3 metric tuples per card with non-empty labels", () => {
+    // v1.5 phase-5 a11y fix: dropped the empty-string filler "—"/"" tuple from
+    // the audit-log card so the rendered <dl> has no zero-content <dd>. Cards
+    // must still have at least one metric (otherwise the grid collapses) and
+    // every metric needs a real label (axe `definition-list` rule).
     const cards = buildCards(mockOverview);
     for (const card of cards) {
-      expect(card.metrics).toHaveLength(3);
+      expect(card.metrics.length).toBeGreaterThanOrEqual(1);
+      expect(card.metrics.length).toBeLessThanOrEqual(3);
+      for (const metric of card.metrics) {
+        expect(metric.label.trim()).not.toBe("");
+        expect(metric.label).not.toBe("—");
+      }
     }
   });