feat: safe HTML descriptions, custom reminder picker, synced default event duration

- Render event descriptions as sanitized HTML (links/line breaks) instead of
  raw escaped text; no script execution
- Reminder picker: presets + custom number+unit (minutes/hours/days/weeks)
- Grey out + hint the reminder editor when a calendar's notifications are off
  (reminders are kept, just not fired)
- New synced setting default_event_duration_minutes (default 60) for new events

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/index.html

@@ -375,6 +375,7 @@
       </div>
       <div class="form-group" id="ev-reminders-group" style="display:none">
         <label data-i18n="reminders">Benachrichtigungen</label>
+        <div id="ev-reminders-hint" class="form-hint" style="display:none" data-i18n="reminders_calendar_off">Für diesen Kalender sind Benachrichtigungen deaktiviert – Erinnerungen werden nicht ausgeführt.</div>
         <div id="ev-reminders-list" class="ev-reminders-list"></div>
         <button type="button" class="btn btn-ghost btn-sm" id="ev-reminder-add" data-i18n="reminder_add">Benachrichtigung hinzufügen</button>
       </div>
@@ -843,6 +844,18 @@
               <span data-i18n="settings_dim_past">Vergangene Termine ausgrauen</span>
             </label>
           </div>
+          <div class="form-group">
+            <label data-i18n="settings_default_duration">Standard-Termindauer</label>
+            <select id="cfg-default-duration">
+              <option value="15">15 min</option>
+              <option value="30">30 min</option>
+              <option value="45">45 min</option>
+              <option value="60">1 h</option>
+              <option value="90">1.5 h</option>
+              <option value="120">2 h</option>
+              <option value="240">4 h</option>
+            </select>
+          </div>
 
           <h4 class="panel-title" style="margin-top:24px" data-i18n="settings_hidden_cals">Ausgeblendete Kalender</h4>
           <div id="hidden-cals-list"><span style="font-size:13px;color:var(--text-3)" data-i18n="settings_no_hidden_cals">Keine ausgeblendeten Kalender</span></div>