dashboard: convert the last dead widgets, and delete the one that had nothing

Three plugins still declared widgets naming Vue components nobody wrote.
Converting them honestly meant three different answers, not one.

notifications gets a real card: the active notifications themselves, not a
count. "4 active" tells an admin nothing; knowing WHICH message the shop is
looking at is the point, and it is how a stale one gets noticed and taken down.

machines gets machines out of service - anything not In Use, excluding
Inventory, because a spare on a shelf is stock rather than a problem. Someone is
supposed to be chasing each of those and today they are visible only to whoever
thinks to filter the list by status.

network gets NOTHING, and its declaration is deleted rather than converted.
Network devices carry no live status - no polling, no reachability check,
nothing that can be wrong - so the only possible card is a count of how many
exist, which is precisely the always-true number this dashboard exists to get
away from. A comment records that, so the next person does not re-add it. If
reachability is ever collected, that is the card.

Also adds a contract test over every declared card: no component names, a valid
renderer and severity, and - the one that matters - the endpoint must be a REAL
route. A declaration pointing at a route nobody wrote is exactly how the old
widgets rotted unnoticed for months, and now it fails the build instead.

CHANGELOG.md

@@ -24,8 +24,9 @@ ADR-007 and ADR-002.
   four, so the reader had to hunt for the problem inside the row. The report
   exists to answer "what needs replacing".
 - Dashboard cards for enforcement failures, machine numbers claimed by two PCs,
-  PCs not reporting, backups that have stopped, printer supplies, and expiring
-  warranties. The dashboard now renders cards plugins declare, which it never
+  PCs not reporting, backups that have stopped, printer supplies, expiring
+  warranties, machines out of service, and the notifications currently showing
+  on the floor. The dashboard now renders cards plugins declare, which it never
   did before - five plugins had been declaring widgets into a void, pointing at
   components nobody wrote. Cards are ordered by severity, hide themselves when
   there is nothing to report, gate on a permission, and each fetches