Web: custom calendar dropdown in the event editor (shows group icon)

A native <select> can't render the group icon SVG, so the calendar picker
is replaced with a styled dropdown: colour dot + group icon + name, grouped
by account/source with a "Groups" section. The native <select> is kept
hidden as the value holder + change-event source, so all existing save and
private/reminder-row logic is unchanged. Click-outside / Escape close it.

version v46 -> v47.

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

frontend/js/version.js

@@ -1,2 +1,2 @@
 // Increment APP_VERSION with every code change
-export const APP_VERSION = 'v46';
+export const APP_VERSION = 'v47';