feat(filter): richer attribute picker with icons, groups and search

Add a leading icon and grouped, searchable sections to the attribute
picker used by conversation filters, contact filters/segments and
automation rule conditions. FilterSelect gains a `searchable` option
(type-ahead + empty state) and a shared helper supplies per-attribute
icons and section headers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/javascript/dashboard/i18n/locale/en/advancedFilters.json

@@ -19,6 +19,8 @@
       "OR": "OR"
     },
     "INPUT_PLACEHOLDER": "Enter value",
+    "SEARCH_PLACEHOLDER": "Search…",
+    "NO_RESULTS": "No matches",
     "CONTACT_SEARCH_PLACEHOLDER": "Search contacts",
     "CONTACT_FALLBACK": "Contact #{id}",
     "OPERATOR_LABELS": {