Web: reminders UI, per-calendar reminder toggle, popup/grid polish

- Event popup: new "Reminders" section for local events — show/add/remove
  multiple reminders, prefilled with the global default on new events; sent
  in the local create/update body and synced like any other event field.
- Sidebar calendar list: per-calendar reminder bell toggle (PUTs the new
  reminders_enabled flag); disabled state shown dimmed.
- Date picker: a day click now commits immediately in date-only mode instead
  of requiring an extra "Save" click.
- Start/End: moving the end before the start now shifts the start back
  (mirroring the start handler) instead of showing an error toast.
- Calendar dropdown: group calendars moved under a "Groups" optgroup; dropped
  the 👥 emoji prefix (a native <select> can't render the icon SVG).
- Month grid: selectedDate defaults to today so the highlight no longer
  follows the cells while navigating before any click.
- Today cell tint now derives from --today-color instead of hardcoded blue.
- version v44 -> v45.

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

frontend/index.html

@@ -364,6 +364,11 @@
           <div class="ev-color-preview" id="ev-color-preview" title="Farbe wählen"></div>
         </div>
       </div>
+      <div class="form-group" id="ev-reminders-group" style="display:none">
+        <label data-i18n="reminders">Benachrichtigungen</label>
+        <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>
     </div>
     <div class="modal-footer">
       <button class="btn btn-danger hidden" id="ev-delete">Löschen</button>