swarm-ui: self-updating RelativeTime component

Closes #3445.

New ui/relative-time/RelativeTime — takes a UTC epoch-ms instant, not
a precomputed age, and re-renders itself on a 1s interval so a tab
left open doesn't silently show a frozen 'fresh (5s ago)' hours
later. Pauses while the document is hidden, resyncs immediately on
becoming visible again.

StatusChip's label widened from string to ComponentChildren so a
chip can embed the live ticker (a plain string couldn't carry it).
HivesPage's freshness column now derives the age from last_seen_unix
client-side via RelativeTime instead of rendering the once-computed
age_seconds from the API response.

Verified: tsc clean, build succeeds, screenshotted the components
page (ticking observed advancing within one virtual-time-budget
window) and the hives page (fresh/stale/never-reported all render
correctly).

frontend/packages/swarm-ui/src/pages/ComponentsPage.tsx

@@ -8,6 +8,7 @@
 import { useState } from 'preact/hooks';
 import type { ComponentChildren } from 'preact';
 import { Panel } from '../ui/panel/Panel.js';
+import { RelativeTime } from '../ui/relative-time/RelativeTime.js';
 import { StatusChip, type ChipTone } from '../ui/status-chip/StatusChip.js';
 import { Table, type TableColumn } from '../ui/table/Table.js';
 import { TextField } from '../ui/text-field/TextField.js';
@@ -115,6 +116,17 @@ export function ComponentsPage() {
         </Sample>
       </Section>
 
+      <Section title="RelativeTime">
+        <div class="components-chip-row">
+          <Sample label="5s ago, ticking">
+            <RelativeTime epochMs={Date.now() - 5_000} />
+          </Sample>
+          <Sample label="2h ago, ticking">
+            <RelativeTime epochMs={Date.now() - 2 * 3_600_000} />
+          </Sample>
+        </div>
+      </Section>
+
       <Section title="TextField">
         <Sample label="editable">
           <TextFieldSample />