feat(tasks): assign tasks in shared lists, unified people search, list avatars

- Tasks in shared task lists can be assigned to one or more people with access
  (task_assignees); assignments are pruned whenever access is lost
- UserAutocomplete: type-to-search everywhere, nothing before typing, max 3 results,
  first match preselected
- Task list and list tabs: avatar stack of members and round monochrome counter
- Shared list items remember who added them (list_items.created_by)
- Doku/ui-patterns.md documents the shared UI patterns

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/index.css

@@ -293,6 +293,22 @@ body {
     font-weight: 500;
   }
 
+  /* Round count bubble: same shape as the nav bar counters, but monochrome (tabs of task lists & lists) */
+  .badge-count {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    min-width: 16px;
+    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;
+  }
+
   .badge-ok {
     display: inline-flex;
     align-items: center;