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/components/MobileHeaderMenu.tsx

@@ -2,6 +2,7 @@ import React from 'react';
 import { Check, X } from 'lucide-react';
 import { useMobileActions } from '../context/MobileActionContext';
 import { triggerHaptic } from '../lib/api';
+import { CountBadge } from './CountBadge';
 
 interface MobileHeaderMenuProps {
   activeTabLabel: string;
@@ -109,7 +110,9 @@ export const MobileHeaderMenu: React.FC<MobileHeaderMenuProps> = ({ activeTabLab
                         </div>
 
                         <div className="flex items-center gap-2 flex-shrink-0">
-                          {item.badge !== undefined && item.badge !== '' && (
+                          {typeof item.badge === 'number' && !isAccent ? (
+                            <CountBadge count={item.badge} showZero variant={isActive ? 'accent' : 'muted'} />
+                          ) : item.badge !== undefined && item.badge !== '' && (
                             <span
                               className={`px-2 py-0.5 rounded-full text-xs font-mono font-semibold ${
                                 isAccent