Add a Domains widget to the dashboard

A Domains card completing the Overview row (DNS, Secrets, Servers,
Domains), shaped like the Secrets card: a status badge (N expired / N
expiring / All OK / No expiry dates / Not configured), tracked, expiring
and expired counts, an OK/expiring/expired/no-date bar, and details --
the expired domains and the soonest-expiring ones (first three, soonest
first), the next one to expire when everything is fine, and a note when
some domains couldn't be refreshed, linking to the Domains page.

Purely front end: the existing /api/domains already carries each domain's
status, days left and last-check error, computed against the warning
window from Settings. A registry that doesn't publish expiry dates (.de,
.eu) shows as "no date" and is not counted as a failed refresh; anything
else that stopped a refresh is.

Verified in a browser against the real domains router across four states:
problems (expired, several expiring, an undated one, a failed refresh),
all healthy, only undated domains, and none tracked. Web build only; no
backend or database changes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -32,6 +32,9 @@ All modules from the original plan are built:
   usage threshold — decided by the same rules as the health alerts, so the
   widget and the notifications always agree, and following the thresholds in
   Settings even for viewers who can't open them.
+  The Domains widget shows how many registrations are tracked, which are
+  expired or expiring (soonest first), the next one to expire, and how many
+  couldn't be refreshed.
 - **Diagnostic Log** (admin-only) — every call this app makes to a DNS
   provider or integration (Tailscale, Proxmox, Synology, Semaphore, Gitea,
   Dockhand), success or failure, with latency and the error message if it