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/themes/theme-template.css

@@ -8,8 +8,9 @@
   /* --- 1. AKZENTFARBE (Buttons, aktive Tabs, Fokus-Ränder, Badges) --- */
   --accent-primary: #2563eb;
   --accent-hover: #3b82f6;
-  --accent-dim: rgba(37, 99, 235, 0.14);
-  --accent-border: rgba(59, 130, 246, 0.45);
+  /* Tints follow --accent-primary automatically; set fixed values only if you want different ones */
+  --accent-dim: color-mix(in srgb, var(--accent-primary) 14%, transparent);
+  --accent-border: color-mix(in srgb, var(--accent-hover) 45%, transparent);
   --text-on-accent: #ffffff;
 
   /* --- 2. OBERFLÄCHEN & HINTERGRÜNDE --- */