feat: add models cache store and fix search placeholder truncation

- Add useModelsStore with 30s cache for model list to reduce API calls
  - Refactor SystemPage to use cached models from store
  - Shorten ConfigPage search placeholder to prevent text truncation

src/stores/index.ts

@@ -7,3 +7,4 @@ export { useThemeStore } from './useThemeStore';
 export { useLanguageStore } from './useLanguageStore';
 export { useAuthStore } from './useAuthStore';
 export { useConfigStore } from './useConfigStore';
+export { useModelsStore } from './useModelsStore';