feat(i18n): add English + Chinese localization

The app had zero localization ("软件多语言做了吗" — no). Add Flutter's
official gen-l10n pipeline and localize the core flow the user sees.

- pubspec: flutter_localizations + intl + generate: true
- l10n.yaml + lib/l10n/app_en.arb + app_zh.arb (37 strings)
- main.dart: localizationsDelegates + supportedLocales (follows OS locale)
- pen editor: all tool tooltips, page pill, error states localized
- home: app bar actions + empty-state buttons localized

Proven end-to-end: l10n_test pumps the same widget under Locale('en')
and Locale('zh') and asserts English vs Chinese strings resolve.

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

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

lib/l10n/app_zh.arb

@@ -0,0 +1,37 @@
+{
+  "@@locale": "zh",
+  "appTitle": "BadNote",
+  "settings": "设置",
+  "search": "搜索",
+  "importPdf": "导入 PDF",
+  "importPpt": "导入 PPT",
+  "penCanvasBeta": "手写画布（测试版）",
+  "newNote": "新建笔记",
+  "open": "打开",
+  "cancel": "取消",
+  "delete": "删除",
+  "deleteNoteTitle": "删除笔记？",
+  "deleteNote": "删除笔记",
+  "openInSplitView": "分屏打开",
+  "splitViewSubtitle": "PDF 参考 + 草稿纸",
+  "removeDocument": "移除文档",
+  "processingPptx": "正在处理 PPTX…",
+  "processingPresentation": "正在处理演示文稿…",
+  "couldNotOpenPresentation": "无法打开演示文稿。",
+  "toolPen": "钢笔",
+  "toolHighlighter": "荧光笔",
+  "toolEraser": "橡皮擦",
+  "actionUndo": "撤销",
+  "actionRedo": "重做",
+  "fingerDrawingOn": "手指书写：开",
+  "fingerDrawingOff": "手指书写：关（仅手写笔）",
+  "pages": "页面",
+  "penSettings": "手写笔设置",
+  "inputDiagnostic": "输入诊断（写入日志文件）",
+  "back": "返回",
+  "previousPage": "上一页",
+  "nextPage": "下一页",
+  "failedToOpenPdf": "打开 PDF 失败：\n{error}",
+  "pdfNoPages": "PDF 没有任何页面。",
+  "pageOfPages": "{current} / {total}"
+}