Add a Servers widget to the dashboard

A Servers card in the dashboard's Overview row, in the same shape as the
DNS and Secrets cards: a status badge (N offline / N disks nearly full /
All online / No agent data / Not configured), total, online and offline
counts, an online/offline/no-data bar, and the details that matter --
which servers are offline and for how long (the first three, linking to
their pages), which disks are at or above the usage threshold, and a
Linux/Windows split when there are Windows machines.

The numbers come from a new GET /api/servers/summary, computed on the
server with the health monitor's own evaluateHealth. That means "offline"
and "disk full" are decided by exactly the rules the alerts use, so the
widget can't say a server is fine while an alert says it isn't, and it
follows the thresholds set in Settings, which viewers can't read
themselves and so couldn't have applied client-side. Servers under a
maintenance window are marked as such. A server that has never reported
(no agent, or tracked only through Proxmox) counts as "no data" rather
than offline, an offline server's stale disk figure isn't reported, and a
garbled report doesn't blank the widget.

Readable by every signed-in user, like the dashboard itself.

Verified with 23 backend checks (offline/online/no-data classification
including the bare SQLite timestamp format, ordering, colons in Windows
mounts, threshold changes, maintenance flags, agreement with the alert
path's own offline and disk sets, empty and garbled inputs, and the route
as a viewer following Settings) and in a browser against the real router
across five states: problems, disks only, all healthy, no data, and empty.
Real dev database mtime untouched.

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

README.md

@@ -27,6 +27,11 @@ All modules from the original plan are built:
   alongside its disk-health breakdown. Breakdowns render as a stacked
   proportion bar with a legend — no charting
   library, matching the rest of the app's plain-Tabler-CSS approach.
+  The Servers widget shows how many servers are online, offline, or have never
+  reported, which are offline and for how long, and any disk at or above the
+  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.
 - **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