feat(i18n): localize settings + search screens

Extend the en/zh localization to the two screens reached from the home
app bar (the "全都用 + 多语言" ask):
- settings: title, theme-mode segments (System/Light/Dark), seed-color
  description, color-picker + clear-data dialogs.
- search: hint, error, empty/no-results states, Notes/Documents section
  headers, page label.

New ARB keys regenerated; l10n_test now asserts the new keys resolve in
both English and Chinese.

flutter analyze: 0 issues. l10n_test: 3/3.

lib/l10n/app_zh.arb

@@ -15,6 +15,22 @@
   "openInSplitView": "分屏打开",
   "splitViewSubtitle": "PDF 参考 + 草稿纸",
   "removeDocument": "移除文档",
+  "ok": "确定",
+  "pickColor": "选择颜色",
+  "clearSettingsTitle": "清除所有本地设置？",
+  "clear": "清除",
+  "settingsReset": "设置已恢复默认",
+  "themeSystem": "跟随系统",
+  "themeLight": "浅色",
+  "themeDark": "深色",
+  "seedColorDesc": "Material 3 主题种子色",
+  "searchHint": "搜索笔记和文档…",
+  "searchError": "搜索出错：{error}",
+  "noResultsFor": "没有“{query}”的结果",
+  "typeToSearch": "输入以搜索你的笔记和文档",
+  "sectionNotes": "笔记",
+  "sectionDocuments": "文档",
+  "pageLabel": "第 {page} 页",
   "processingPptx": "正在处理 PPTX…",
   "processingPresentation": "正在处理演示文稿…",
   "couldNotOpenPresentation": "无法打开演示文稿。",