Fix bulk assign user search: replace broken combobox with typeahead

The combobox modal used lt-combobox-list but lt.combobox looks for
lt-combobox-dropdown — it returned immediately, wiring nothing.

Replaced with lt.typeahead which is correct for single-select search:
- Filters users client-side as you type (minChars:1, debounced 150ms)
- Shows display_name (username) with highlight on match
- onSelect stores user ID and shows "✓ Name" confirmation below input
- Input auto-focuses when modal opens
- Enter key now selects first result even without arrow-key navigation
  (same fix applied to lt.combobox Enter handler)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/js/base.js

@@ -2065,7 +2065,7 @@
         if (!dropdown.classList.contains('is-open')) return;
         if (e.key === 'ArrowDown')  { e.preventDefault(); _moveFocus(1); }
         if (e.key === 'ArrowUp')    { e.preventDefault(); _moveFocus(-1); }
-        if (e.key === 'Enter')      { e.preventDefault(); if (_focusedIdx >= 0 && _items[_focusedIdx]) _select(_items[_focusedIdx]); }
+        if (e.key === 'Enter')      { e.preventDefault(); const idx = _focusedIdx >= 0 ? _focusedIdx : 0; if (_items[idx]) _select(_items[idx]); }
         if (e.key === 'Escape')     { dropdown.classList.remove('is-open'); }
         if (e.key === 'Tab')        { dropdown.classList.remove('is-open'); }
       });