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>

e2e/share.e2e.mjs

@@ -69,8 +69,7 @@ const main = async () => {
   await search.click();
   await search.type('user');
   await sleep(300);
-  await search.press('ArrowDown');
-  await search.press('Enter');
+  await search.press('Enter'); // the first match is preselected
   const roleSelect = A.page.locator('select').filter({ has: A.page.locator('option[value="editor"]') }).last();
   check((await roleSelect.isEnabled()) && (await roleSelect.isVisible()), 'Rollenauswahl ist im Dialog verfügbar');
   await roleSelect.selectOption('editor');