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>
+  );
+};