dashboard: PCs not reporting, and the card styling standard it broke

Second wave-one card. GET /api/computers/dashboard/quiet lists two populations
and deliberately does not merge them into one count. A PC that reported and
went quiet is probably off, moved or broken. A PC that has NEVER reported is
worse: not enrolled, or enrolled against the wrong pc-type, so nothing enforces
anything on it and no backup of it exists. That one hides indefinitely because
nothing about it fails loudly - the same shape as the bay that carried a wrong
machine number for weeks.

Never-reported sorts above the merely quiet, then longest silence first: the
order someone should work down the list, not the order rows left the table. A
soft-deleted PC is excluded - a decommissioned machine is silent on purpose,
and listing it would train people to ignore the card, which is the failure this
whole board exists to avoid.

The window is computers_quietreporthours, default 24, because every site will
disagree with any number picked here (ADR-015). A malformed value falls back
rather than failing the card.

This also replaces the computers plugin's old widget declaration, which named a
component nobody ever wrote. Four such declarations remain and will convert as
their cards arrive.

Two fixes to the renderer found while wiring this up. Meta specs now support a
trailing unit, so a row reads 'quiet for 3 days' rather than 'quiet for 3'. And
the card styles hardcoded hex colours against the frontend standard, including
a var(--card-bg) that DOES NOT EXIST - the variable is --bg-card - so the
fallback would have painted every card white and broken dark mode entirely.
Now --bg-card, --border, --danger, --warning, --primary and --link throughout.

frontend/src/components/dashboardCards.spec.js

@@ -53,6 +53,15 @@ describe('mapping a row', () => {
     expect(row.link).toBe('/pcs/42')
   })
 
+  it('supports a trailing unit so the text reads as a sentence', () => {
+    const card = {
+      render: 'exceptions',
+      map: { title: 'hostname', meta: [{ key: 'quietdays', label: 'quiet for', suffix: ' days' }] },
+      _data: [{ hostname: 'QUIETPC', quietdays: 3 }],
+    }
+    expect(cardRows(card)[0].meta[0].text).toBe('quiet for 3 days')
+  })
+
   it('drops empty meta values instead of rendering a stray label', () => {
     const card = { ...failuresCard, _data: [{ hostname: 'X', message: '', exitcode: null }] }
     expect(cardRows(card)[0].meta).toEqual([])