feat(i18n): full RTL/CJK support — fonts, Japanese locale, directional fixes

Bundle Noto Sans Arabic + Noto Sans JP (SIL OFL 1.1) as per-glyph fallbacks
for both the UI text theme and every generated PDF, so RTL (Arabic) and CJK
render on all platforms — including desktop, where the system font may lack
them — instead of tofu. A shared pdf_fonts.dart centralizes the PDF theme and
the three label/catalog/recovery services reuse it.

Add Japanese (ja) as the CJK reference locale — fittingly, the app's own name
is Japanese (種, tane, 'seed'). Core strings translated; the rest fall back to
English via slang. Wired into supportedLocales (automatic) and the settings
picker.

Fix three physical Alignment.centerLeft -> AlignmentDirectional.centerStart in
variety_detail_screen, and make month-abbreviation and avatar-initial
truncation grapheme-safe (.characters) so CJK text is never cut mid-character.

Tests: CJK/Arabic PDF glyph render, ja locale resolution + English fallback.
The chat-bubble mirroring flagged by the audit was verified correct (already
respects Directionality) and left unchanged.

apps/app_seeds/test/services/i18n_ja_locale_test.dart

@@ -0,0 +1,31 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/i18n/strings.g.dart';
+
+/// Japanese (`ja`) is the CJK reference locale — fitting, since the app's own
+/// name is Japanese (種, *tane*, "seed"). It proves the CJK path end to end:
+/// the locale is wired, core strings resolve in Japanese, and untranslated
+/// keys fall back to the English base (slang `fallback_strategy: base_locale`).
+void main() {
+  test('ja is a supported locale', () {
+    expect(
+      AppLocaleUtils.supportedLocales.map((l) => l.languageCode),
+      contains('ja'),
+    );
+  });
+
+  test('core strings resolve in Japanese', () {
+    final ja = AppLocale.ja.buildSync();
+    expect(ja.menu.inventory, '在庫');
+    expect(ja.common.save, '保存');
+    expect(ja.settings.langJa, '日本語');
+  });
+
+  test('untranslated keys fall back to the English base', () {
+    final ja = AppLocale.ja.buildSync();
+    final en = AppLocale.en.buildSync();
+    // `menu.plantares` is left untranslated in ja (a coined feature name), so
+    // it must surface the English text rather than a blank or a key.
+    expect(ja.menu.plantares, en.menu.plantares);
+    expect(ja.menu.plantares, isNotEmpty);
+  });
+}

apps/app_seeds/test/services/label_sheet_service_test.dart

@@ -76,6 +76,27 @@ void main() {
     expect(String.fromCharCodes(bytes.take(5)), '%PDF-');
   });
 
+  test('buildPdf embeds CJK and Arabic glyphs (no tofu)', () async {
+    // Japanese (CJK) and Arabic (RTL) fall outside DejaVu's coverage; the Noto
+    // fallbacks in the PDF theme must render them without throwing. Guards
+    // against a regression that drops the fallback fonts.
+    final service = LabelSheetService(files: _RecordingFileService());
+    final bytes = await service.buildPdf(
+      labels: const [
+        LabelSheetLabel(
+          varietyLabel: '日本の在来種のトマト',
+          commonName: 'トマト',
+          scientificName: 'Solanum lycopersicum',
+          qrData: 'tane://seed?v=tomato',
+        ),
+        LabelSheetLabel(varietyLabel: 'طماطم بلدية', qrData: 'tane://seed?v=x'),
+      ],
+      format: LabelSheetFormat.cards,
+    );
+    expect(String.fromCharCodes(bytes.take(5)), '%PDF-');
+    expect(bytes.length, greaterThan(1000));
+  });
+
   test('saveLabels hands the PDF to the save dialog', () async {
     final files = _RecordingFileService();
     final service = LabelSheetService(files: files);