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/js/i18n.js

@@ -94,6 +94,14 @@ const translations = {
     private_visibility_hidden: 'Ausblenden',
     created_by: 'Erstellt von: {name}',
     event_private: 'Privat',
+    reminders: 'Benachrichtigungen',
+    reminder_add: 'Benachrichtigung hinzufügen',
+    reminder_at_start: 'Zur Startzeit',
+    reminder_min: 'vor {n} Min.',
+    reminder_hour: 'vor {n} Std.',
+    reminder_day: 'vor {n} Tag(en)',
+    calendar_reminders_on: 'Benachrichtigungen aktiviert',
+    calendar_reminders_off: 'Benachrichtigungen deaktiviert',
     share: 'Teilen',
     import: 'Importieren',
     export: 'Exportieren',
@@ -367,6 +375,14 @@ const translations = {
     private_visibility_hidden: 'Hide completely',
     created_by: 'Created by: {name}',
     event_private: 'Private',
+    reminders: 'Reminders',
+    reminder_add: 'Add reminder',
+    reminder_at_start: 'At start time',
+    reminder_min: '{n} min before',
+    reminder_hour: '{n} h before',
+    reminder_day: '{n} day(s) before',
+    calendar_reminders_on: 'Reminders enabled',
+    calendar_reminders_off: 'Reminders disabled',
     share: 'Share',
     import: 'Import',
     export: 'Export',