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/CountBadge.tsx

@@ -0,0 +1,34 @@
+import React from 'react';
+
+interface CountBadgeProps {
+  count: number | undefined | null;
+  // accent: something waits for you (nav bar); muted: plain amount (tabs, lists)
+  variant?: 'accent' | 'muted';
+  // corner: notification badge on the top right of the nearest `relative` parent
+  position?: 'inline' | 'corner';
+  showZero?: boolean;
+  className?: string; // e.g. to fine-tune the corner offset
+}
+
+// Full class names: Tailwind drops component classes it cannot find literally in the source
+const VARIANT_CLASS = { accent: 'count-badge-accent', muted: 'count-badge-muted' };
+
+// The one counter of the app (Doku/ui-patterns.md); hides itself at 0 unless showZero
+export const CountBadge: React.FC<CountBadgeProps> = ({
+  count,
+  variant = 'muted',
+  position = 'inline',
+  showZero = false,
+  className = '',
+}) => {
+  if (count === undefined || count === null || (count <= 0 && !showZero)) return null;
+  const label = count > 99 ? '99+' : String(count);
+  return (
+    <span
+      className={`count-badge ${VARIANT_CLASS[variant]} ${position === 'corner' ? 'count-badge-corner' : ''} ${className}`}
+      aria-label={String(count)}
+    >
+      {label}
+    </span>
+  );
+};

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

frontend/src/components/__tests__/CountBadge.test.tsx

@@ -0,0 +1,23 @@
+import { describe, it, expect } from 'vitest';
+import { render } from '@testing-library/react';
+import { CountBadge } from '../CountBadge';
+
+describe('CountBadge', () => {
+  it('hides itself at zero unless asked', () => {
+    expect(render(<CountBadge count={0} />).container.firstChild).toBeNull();
+    expect(render(<CountBadge count={undefined} />).container.firstChild).toBeNull();
+    expect(render(<CountBadge count={0} showZero />).container.textContent).toBe('0');
+  });
+
+  it('caps large numbers and keeps the real value for screen readers', () => {
+    const el = render(<CountBadge count={150} />).container.firstChild as HTMLElement;
+    expect(el.textContent).toBe('99+');
+    expect(el.getAttribute('aria-label')).toBe('150');
+  });
+
+  it('maps variant and position to the shared classes', () => {
+    const el = render(<CountBadge count={3} variant="accent" position="corner" />).container.firstChild as HTMLElement;
+    expect(el.className).toContain('count-badge-accent');
+    expect(el.className).toContain('count-badge-corner');
+  });
+});