feat(ui): one CountBadge for all counters; counters on tab corners

- CountBadge component (accent / muted, inline / corner, 99+, showZero);
  used in the nav bar, task list and list tabs, admin organizations and
  numeric badges of the mobile menu; old badge-count removed
- Counters sit on the top right corner of tabs; "Alle Aufgaben" gets one
  (desktop tab and mobile menu)
- Open/done task counters include tasks in shared lists, matching what
  "Alle Aufgaben" shows (nav counter was too low)
- Theme template derives --accent-dim/--accent-border from the accent
- Docs: ui-patterns counters section

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/settings/AdminSettingsView.tsx

@@ -27,6 +27,7 @@ import {
 import { api, setCurrentSharingMode, triggerHaptic } from '../../lib/api';
 import { UserAvatar } from '../../components/UserAvatar';
 import { ToggleSwitch } from '../../components/ToggleSwitch';
+import { CountBadge } from '../../components/CountBadge';
 
 interface AdminSettingsViewProps {
   currentUser: User;
@@ -929,7 +930,7 @@ export const AdminSettingsView: React.FC<AdminSettingsViewProps> = ({
                             <div className="flex items-center gap-2 min-w-0">
                               <Building2 className="w-4 h-4 text-accent shrink-0" />
                               <span className="font-bold text-main truncate">{o.name}</span>
-                              <span className="badge-count">{o.memberCount}</span>
+                              <CountBadge count={o.memberCount} showZero />
                             </div>
                           )}
                           {renamingOrgId !== o.id && (