feat: Admin-Sicht System-Zeit + TZ-Liste in README/.env.example

- /api/settings gibt zusaetzlich timezone, timezone_abbr, server_time,
  ntp_server zurueck (alle read-only, aus Config/ENV).
- AdminView zeigt neuen Abschnitt "System-Zeit" mit Zeitzone, aktueller
  Server-Zeit und NTP-Server samt Hinweis "wird in der .env festgelegt".
- .env.example: Liste gaengiger TZ-Werte mit Link zur IANA-Vollliste.
- README.md: neuer Abschnitt "Zeitzone & NTP" mit Werte-Tabelle.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/AdminView.vue

@@ -37,6 +37,28 @@
       </div>
     </div>
 
+    <!-- System-Info: Zeitzone & NTP (read-only) -->
+    <div class="admin-section">
+      <h3>System-Zeit</h3>
+      <p class="hint">Wird in der <code>.env</code> festgelegt (Keys <code>TZ</code> und <code>NTP_SERVER</code>).
+        Aenderungen erfordern einen Neustart des Backends.</p>
+      <div class="sysinfo">
+        <div class="sysinfo-row">
+          <span class="sysinfo-label">Zeitzone:</span>
+          <code>{{ settings.timezone || '—' }}</code>
+          <span v-if="settings.timezone_abbr" class="sysinfo-extra">({{ settings.timezone_abbr }})</span>
+        </div>
+        <div class="sysinfo-row">
+          <span class="sysinfo-label">Aktuelle Server-Zeit:</span>
+          <code>{{ formatServerTime(settings.server_time) }}</code>
+        </div>
+        <div class="sysinfo-row">
+          <span class="sysinfo-label">NTP-Server:</span>
+          <code>{{ settings.ntp_server || '(deaktiviert)' }}</code>
+        </div>
+      </div>
+    </div>
+
     <!-- System Email -->
     <div class="admin-section">
       <h3>System-E-Mail (SMTP)</h3>
@@ -551,6 +573,17 @@ const smtpForm = ref({
 const smtpPasswordSet = ref(false)
 const onlyofficeConfigured = ref(false)
 const onlyofficeUrl = ref('')
+const settings = ref({ timezone: '', timezone_abbr: '', server_time: '', ntp_server: '' })
+
+function formatServerTime(iso) {
+  if (!iso) return '—'
+  try {
+    return new Date(iso).toLocaleString('de-DE', {
+      day: '2-digit', month: '2-digit', year: 'numeric',
+      hour: '2-digit', minute: '2-digit', second: '2-digit',
+    })
+  } catch { return iso }
+}
 const smtpTesting = ref(false)
 
 // Backup & Restore
@@ -660,6 +693,12 @@ async function loadSettings() {
     smtpPasswordSet.value = res.data.system_smtp_password_set
     onlyofficeConfigured.value = res.data.onlyoffice_configured
     onlyofficeUrl.value = res.data.onlyoffice_url || ''
+    settings.value = {
+      timezone: res.data.timezone || '',
+      timezone_abbr: res.data.timezone_abbr || '',
+      server_time: res.data.server_time || '',
+      ntp_server: res.data.ntp_server || '',
+    }
   } catch { /* first load, defaults */ }
 }
 
@@ -1216,6 +1255,12 @@ onMounted(() => {
 .field-row { display: flex; gap: 0.75rem; align-items: flex-end; }
 .flex-grow { flex: 1; }
 .hint { font-size: 0.85rem; color: var(--p-text-muted-color); margin: 0 0 0.75rem; }
+.hint code { background: var(--p-surface-100); padding: 0.05rem 0.35rem; border-radius: 3px; font-size: 0.8rem; }
+.sysinfo { display: flex; flex-direction: column; gap: 0.4rem; font-size: 0.875rem; }
+.sysinfo-row { display: flex; gap: 0.5rem; align-items: center; flex-wrap: wrap; }
+.sysinfo-label { min-width: 180px; color: var(--p-text-muted-color); }
+.sysinfo code { background: var(--p-surface-100); padding: 0.15rem 0.5rem; border-radius: 4px; }
+.sysinfo-extra { color: var(--p-text-muted-color); font-size: 0.8rem; }
 .invite-section { margin-top: 1.5rem; padding-top: 1rem; border-top: 1px solid var(--p-surface-200); }
 .invite-section h4 { margin: 0 0 0.25rem; font-size: 0.95rem; }
 .invite-row { display: flex; gap: 0.5rem; align-items: flex-start; }