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.

plugins/computers/plugin.py

@@ -163,6 +163,14 @@ class ComputersPlugin(BasePlugin):
         site that installed an earlier one.
         """
         return [
+            {
+                'key': 'computers_quietreporthours',
+                'value': '24',
+                'valuetype': 'integer',
+                'category': 'computers',
+                'description': 'Hours without a collector report before a PC '
+                               'is listed as not reporting on the dashboard.',
+            },
             {
                 'key': 'computers_machinelink_alerts',
                 'value': 'false',
@@ -1247,14 +1255,30 @@ class ComputersPlugin(BasePlugin):
         return [computerscli]
 
     def get_dashboard_widgets(self) -> List[Dict]:
-        """Return dashboard widget definitions."""
+        """Dashboard cards this plugin contributes.
+
+        REPLACES a declaration that named a Vue component which was never
+        written - as did four other plugins - so nothing rendered and nobody
+        noticed, because the frontend never called the widgets endpoint either.
+        The contract now carries data and a renderer; core draws it.
+        """
         return [
             {
-                'name': 'Computer Status',
-                'component': 'ComputerStatusWidget',
-                'endpoint': '/api/computers/dashboard/summary',
-                'size': 'medium',
-                'position': 6,
+                'id': 'computers-quiet',
+                'title': 'PCs not reporting',
+                'endpoint': '/api/computers/dashboard/quiet',
+                'render': 'exceptions',
+                'severity': 'warning',
+                'permission': 'computers.view',
+                'empty': 'hide',
+                'position': 20,
+                'map': {
+                    'title': 'hostname',
+                    'detail': 'state',
+                    'meta': [{'key': 'quietdays', 'label': 'quiet for',
+                              'suffix': ' days'}],
+                    'link': '/pcs/{computerid}',
+                },
             },
         ]