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/date-picker.js

@@ -148,6 +148,9 @@ export function openDatePicker(anchor, value, mode = 'datetime') {
       card.querySelectorAll('.dtp-day').forEach(el => {
         el.addEventListener('click', () => {
           selDate = new Date(parseInt(el.dataset.ts));
+          // In date-only mode there's nothing else to choose — commit the day
+          // immediately instead of forcing an extra "Save" click.
+          if (mode === 'date') { done(buildResult()); return; }
           if (el.classList.contains('other')) {
             viewYear  = selDate.getFullYear();
             viewMonth = selDate.getMonth();