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_en.arb

@@ -0,0 +1,46 @@
+{
+  "@@locale": "en",
+  "appTitle": "BadNote",
+  "settings": "Settings",
+  "search": "Search",
+  "importPdf": "Import PDF",
+  "importPpt": "Import PPT",
+  "penCanvasBeta": "Pen Canvas (beta)",
+  "newNote": "New Note",
+  "open": "Open",
+  "cancel": "Cancel",
+  "delete": "Delete",
+  "deleteNoteTitle": "Delete note?",
+  "deleteNote": "Delete note",
+  "openInSplitView": "Open in Split View",
+  "splitViewSubtitle": "PDF reference + scratchpad",
+  "removeDocument": "Remove document",
+  "processingPptx": "Processing PPTX...",
+  "processingPresentation": "Processing presentation...",
+  "couldNotOpenPresentation": "Could not open presentation.",
+  "toolPen": "Pen",
+  "toolHighlighter": "Highlighter",
+  "toolEraser": "Eraser",
+  "actionUndo": "Undo",
+  "actionRedo": "Redo",
+  "fingerDrawingOn": "Finger drawing ON",
+  "fingerDrawingOff": "Finger drawing OFF (pen only)",
+  "pages": "Pages",
+  "penSettings": "Pen settings",
+  "inputDiagnostic": "Input diagnostic (writes a log file)",
+  "back": "Back",
+  "previousPage": "Previous page",
+  "nextPage": "Next page",
+  "failedToOpenPdf": "Failed to open PDF:\n{error}",
+  "@failedToOpenPdf": {
+    "placeholders": { "error": { "type": "String" } }
+  },
+  "pdfNoPages": "PDF has no pages.",
+  "pageOfPages": "{current} / {total}",
+  "@pageOfPages": {
+    "placeholders": {
+      "current": { "type": "int" },
+      "total": { "type": "int" }
+    }
+  }
+}