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/lib/i18n/fr.i18n.json

@@ -98,6 +98,7 @@
     "langAst": "Asturianu",
     "langFr": "Français",
     "langDe": "Deutsch",
+    "langJa": "日本語",
     "about": "À propos",
     "aboutText": "Inventaire local-first et chiffré pour les semences traditionnelles. AGPL-3.0.",
     "aboutOpen": "À propos de Tane"