feat(settings): cross-category settings search (T-450)

A search box (ClideFilterBox) atop the rail filters fields across every
registered category. While searching, the panel swaps to a results view that
groups the matching fields under category subheaders — rendered with the same
carded rows and editable inline — and each rail row shows its match count with
zero-match categories dimmed. Completes the settings-UI infra spine (T-444):
shell, engine, rail, scope tags, search.

Tests: search filters across categories, hides non-matches, and surfaces the
per-category rail count.

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

lib/kernel/src/i18n/catalog/builtin.settings-ui_en_us.json

@@ -4,6 +4,8 @@
   "modal.close.hint": { "translation": "Close settings without changing anything" },
   "rail.header": { "translation": "Categories" },
   "panel.empty": { "translation": "No settings categories are registered yet." },
+  "search.hint": { "translation": "Search settings…" },
+  "search.empty": { "translation": "No settings match your search." },
   "scope.project": { "translation": "This project" },
   "scope.always": { "translation": "All clide" },
   "scope.default": { "translation": "Default" },