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/AuditLog.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useState } from "react";
 import { api, type AuditLogEntry } from "../api/client";
+import { formatDateTime } from "../utils/date";
 
 function targetLabel(e: AuditLogEntry): string {
   if (!e.targetType) return "—";
@@ -47,7 +48,7 @@ export default function AuditLog() {
             <tbody>
               {entries?.map((e) => (
                 <tr key={e.id}>
-                  <td>{new Date(e.createdAt).toLocaleString()}</td>
+                  <td>{formatDateTime(new Date(e.createdAt))}</td>
                   <td>{e.actorLabel ?? "—"}</td>
                   <td>
                     <span className="badge bg-blue-lt">{e.category}</span>