feat(ui): searchable model picker with provider group headers — v0.50.86 (PR #659 by @mmartial)

- Live search input in model dropdown (filter by name or ID)
- Provider group headers preserved in filtered view
- Clear button, Escape-to-close, No models found empty state
- i18n EN/ES/zh-CN strings
- CSS uses var(--accent) consistent with current theme system
- zh-CN double-escape fix included
- Provider headers regression fix included
- 1423 tests pass

Co-authored-by: mmartial <mmartial@users.noreply.github.com>

static/i18n.js

@@ -58,8 +58,9 @@ const LOCALES = {
     provider_mismatch_label: 'Provider mismatch',
     model_custom_label: 'Custom model ID',
     model_custom_placeholder: 'e.g. openai/gpt-5.4',
+    model_search_placeholder: 'Search models…',
+    model_search_no_results: 'No models found',
     // commands.js
-    cmd_help: 'List available commands',
     cmd_clear: 'Clear conversation messages',
     cmd_compress: 'Manually compress conversation context (usage: /compress [focus topic])',
     cmd_compact_alias: 'Legacy alias for /compress',
@@ -481,6 +482,8 @@ const LOCALES = {
     provider_mismatch_label: 'Proveedor incompatible',
     model_custom_label: 'ID de modelo personalizado',
     model_custom_placeholder: 'p. ej. openai/gpt-5.4',
+    model_search_placeholder: 'Buscar modelos…',
+    model_search_no_results: 'No se encontraron modelos',
     // commands.js
     cmd_help: 'Listar los comandos disponibles',
     cmd_clear: 'Borrar los mensajes de la conversación',
@@ -1115,6 +1118,8 @@ const LOCALES = {
     provider_mismatch_label: '\u63d0\u4f9b\u5546\u4e0d\u5339\u914d',
     model_custom_label: '\u81ea\u5b9a\u4e49\u6a21\u578b ID',
     model_custom_placeholder: '\u4f8b\u5982 openai/gpt-5.4',
+    model_search_placeholder: '\u641c\u7d22\u6a21\u578b\u2026',
+    model_search_no_results: '\u672a\u627e\u5230\u6a21\u578b',
     // commands.js
     cmd_help: '\u67e5\u770b\u53ef\u7528\u547d\u4ee4',
     cmd_clear: '\u6e05\u7a7a\u5f53\u524d\u5bf9\u8bdd\u6d88\u606f',