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>

README.md

@@ -49,8 +49,10 @@ All modules from the original plan are built:
 - **Settings** (admin-only) — notification channels (Gotify, ntfy, SMTP,
   generic webhook) with per-channel test buttons, per-event toggles (DNS
   record added/updated/deleted, daily secret-expiry reminder with a
-  configurable time/timezone), and badge-color customization for both DNS
-  providers and integration types.
+  configurable time/timezone), badge-color customization for both DNS
+  providers and integration types, and a date/time display format
+  (date order, 12/24-hour clock) applied consistently to every table in
+  the app.
 
 All six integrations follow the same config-in-UI + encrypted-credentials
 pattern, added (and edited — e.g. to rotate an expired API token without