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);
+  });
+}