Add a Date & Time setting and fix inconsistent date formats app-wide

Different tables used different date formats: Servers & Tasks used a
fixed "YYYY-MM-DD HH:mm:ss" (24h), while Audit Log, DNS, Integrations
(Tailscale/Semaphore), and Users called plain .toLocaleString() with
no options, which renders using the browser's own locale — different
per browser/OS, and inconsistent with the other pages' fixed format.

- New Settings -> Date & Time page: pick date order (YYYY-MM-DD,
  DD/MM/YYYY, MM/DD/YYYY) and 12h vs 24h clock, with a live preview.
- utils/date.ts's formatDateTime() now reads these settings instead of
  being hardcoded to sv-SE/24h; the setting is fetched once at app
  startup (alongside /api/me) via a new non-secret GET
  /api/settings/display (any signed-in user, same rationale as the
  badge-color endpoints) and applied immediately on save too, without
  needing a page reload.
- Switched every remaining raw new Date(...).toLocaleString() call
  (Audit Log, DNS zone sync time, Tailscale/Semaphore last-seen,
  Users' last login) over to the shared formatter, so every table now
  renders dates identically.

Verified the formatter's date-order x 12h logic against all six
combinations plus the midnight/noon 12h edge cases, and the settings
endpoints end-to-end (defaults, partial updates, validation
rejection, audit logging) against the real dev server.

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

web/src/pages/Integrations.tsx

@@ -17,6 +17,7 @@ import {
 } from "../api/client";
 import IntegrationForm from "../components/IntegrationForm";
 import IntegrationEditForm from "../components/IntegrationEditForm";
+import { formatDateTime } from "../utils/date";
 
 function proxmoxStatusBadge(status: string) {
   return status === "running" ? (
@@ -547,7 +548,7 @@ export default function Integrations({ user }: { user: CurrentUser }) {
                               )}
                               {!d.authorized && <span className="badge bg-yellow-lt text-yellow">Unauthorized</span>}
                             </td>
-                            <td className="text-secondary">{d.lastSeen ? new Date(d.lastSeen).toLocaleString() : "—"}</td>
+                            <td className="text-secondary">{d.lastSeen ? formatDateTime(new Date(d.lastSeen)) : "—"}</td>
                             {canEdit && (
                               <td>
                                 <div className="btn-list flex-nowrap">
@@ -775,7 +776,7 @@ export default function Integrations({ user }: { user: CurrentUser }) {
                             <td className="text-secondary">{t.projectName}</td>
                             <td className="text-secondary">
                               {t.lastTask
-                                ? new Date(t.lastTask.end ?? t.lastTask.start ?? t.lastTask.created ?? "").toLocaleString()
+                                ? formatDateTime(new Date(t.lastTask.end ?? t.lastTask.start ?? t.lastTask.created ?? ""))
                                 : "—"}
                             </td>
                             <td>{semaphoreStatusBadge(t)}</td>