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/version.js

@@ -1,2 +1,2 @@
 // Increment APP_VERSION with every code change
-export const APP_VERSION = 'v41';
+export const APP_VERSION = 'v42';