feat(web): group members in the sidebar + per-user member colours

Moved the group member filter from a top bar into the left sidebar (a
"Mitglieder" section that replaces the personal calendar list while a group is
active). Each member row has a checkbox (show/hide their events) and a colour
dot that opens the colour picker to recolour that member just for this viewer —
stored per-device in localStorage and applied over the server colour. Bumped v44.

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

frontend/index.html

@@ -179,6 +179,14 @@
           <div class="mini-cal-days" id="mini-days"></div>
         </div>
 
+        <!-- Group members (shown only in the group overlay; hide/show people) -->
+        <div class="cal-list hidden" id="group-members">
+          <div class="cal-list-header">
+            <span data-i18n="group_members">Mitglieder</span>
+          </div>
+          <div id="group-members-items"></div>
+        </div>
+
         <!-- Calendar List -->
         <div class="cal-list" id="cal-list">
           <div class="cal-list-header">