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/utils/date.ts

@@ -1,12 +1,52 @@
-/** Formats a date as "YYYY-MM-DD HH:mm:ss" on a 24-hour clock. */
+import type { DateFormat, TimeFormat } from "../api/client";
+
+export interface DateTimeSettings {
+  dateFormat: DateFormat;
+  timeFormat: TimeFormat;
+}
+
+let current: DateTimeSettings = { dateFormat: "ymd", timeFormat: "24h" };
+
+/** Called once at app startup (and whenever the setting is changed) so every formatDateTime() call site picks it up without prop-drilling. */
+export function setDateTimeSettings(settings: DateTimeSettings): void {
+  current = settings;
+}
+
+function pad(n: number): string {
+  return String(n).padStart(2, "0");
+}
+
+/** Formats a date using the app-wide date/time display settings (defaults to "YYYY-MM-DD HH:mm:ss" on a 24-hour clock). */
 export function formatDateTime(date: Date): string {
-  return date.toLocaleString("sv-SE", {
-    year: "numeric",
-    month: "2-digit",
-    day: "2-digit",
-    hour: "2-digit",
-    minute: "2-digit",
-    second: "2-digit",
-    hour12: false,
-  });
+  const year = date.getFullYear();
+  const month = pad(date.getMonth() + 1);
+  const day = pad(date.getDate());
+
+  let datePart: string;
+  switch (current.dateFormat) {
+    case "dmy":
+      datePart = `${day}/${month}/${year}`;
+      break;
+    case "mdy":
+      datePart = `${month}/${day}/${year}`;
+      break;
+    case "ymd":
+    default:
+      datePart = `${year}-${month}-${day}`;
+      break;
+  }
+
+  const minutes = pad(date.getMinutes());
+  const seconds = pad(date.getSeconds());
+  let timePart: string;
+  if (current.timeFormat === "12h") {
+    const hour24 = date.getHours();
+    const period = hour24 >= 12 ? "PM" : "AM";
+    const hour12 = hour24 % 12 || 12;
+    timePart = `${hour12}:${minutes}:${seconds} ${period}`;
+  } else {
+    timePart = `${pad(date.getHours())}:${minutes}:${seconds}`;
+  }
+
+  return `${datePart} ${timePart}`;
 }