feat(web): hide individual member calendars in the group view

In the group overlay a per-member filter bar appears under the banner: tap a
member (or the group calendar) to hide/show their events, Outlook-style.
Filtering is client-side by event owner; members + colours load from the group
detail. Bumped version.

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

frontend/js/i18n.js

@@ -124,6 +124,7 @@ const translations = {
     group_created: 'Gruppe erstellt',
     group_view_label: 'Gruppenansicht: {name}',
     group_exit: 'Gruppenansicht verlassen',
+    group_calendar: 'Gruppenkalender',
     group_icon: 'Icon',
     group_visible_flag: 'Für deine Gruppen sichtbar',
     group_new_event: '+ Gruppentermin',
@@ -396,6 +397,7 @@ const translations = {
     group_created: 'Group created',
     group_view_label: 'Group view: {name}',
     group_exit: 'Exit group view',
+    group_calendar: 'Group calendar',
     group_icon: 'Icon',
     group_visible_flag: 'Visible to your groups',
     group_new_event: '+ Group event',