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/index.css

@@ -293,8 +293,9 @@ body {
     font-weight: 500;
   }
 
-  /* Round count bubble: same shape as the nav bar counters, but monochrome (tabs of task lists & lists) */
-  .badge-count {
+  /* Round counter, rendered by <CountBadge> (Doku/ui-patterns.md). Accent = needs attention (nav bar),
+     muted = plain amount (tabs, lists). Corner = notification badge on the top right of a relative parent. */
+  .count-badge {
     display: inline-flex;
     align-items: center;
     justify-content: center;
@@ -302,11 +303,27 @@ body {
     height: 16px;
     padding: 0 0.25rem;
     border-radius: 9999px;
-    background-color: var(--text-muted);
-    color: var(--bg-panel);
     font-size: 9px;
     font-weight: 700;
     line-height: 1;
+    font-variant-numeric: tabular-nums;
+    pointer-events: none;
+  }
+  .count-badge-accent {
+    background-color: var(--accent-primary);
+    color: var(--text-on-accent);
+  }
+  .count-badge-muted {
+    background-color: var(--text-muted);
+    color: var(--bg-panel);
+  }
+  .count-badge-corner {
+    position: absolute;
+    top: -0.5rem;
+    right: -0.5rem;
+    z-index: 1;
+    /* ring in the page colour keeps the badge readable on top of borders */
+    box-shadow: 0 0 0 2px var(--bg-app);
   }
 
   .badge-ok {