feat: add IconGrid, EntityPicker controls and enhance search panel

Port icon grid and entity picker patterns from wled-screen-controller.
IconGrid replaces plain <select> elements with visual icon grids for
known item sets (widget type, icon type, healthcheck method, permission
level). EntityPicker replaces search dropdowns with a command-palette
style overlay with keyboard navigation and filtering.

Enhance SearchDialog with keyboard navigation (arrow keys, Enter,
Escape), grouped results with section headers, active highlight,
and a footer with shortcut hints.

src/lib/i18n/en.json

@@ -258,6 +258,11 @@
   "search.no_results": "No results for \"{query}\"",
   "search.apps": "Apps",
   "search.boards": "Boards",
+  "search.nav_hint": "navigate",
+  "search.select_hint": "select",
+  "search.close_hint": "close",
+
+  "common.search_filter": "Filter...",
 
   "common.save": "Save",
   "common.cancel": "Cancel",