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/ja.i18n.json

@@ -0,0 +1,46 @@
+{
+    "app": {
+        "title": "Tane"
+    },
+    "common": {
+        "save": "保存",
+        "cancel": "キャンセル",
+        "delete": "削除",
+        "edit": "編集",
+        "type": "種類",
+        "comingSoon": "近日公開",
+        "offline": "オフラインです — 共有を一時停止しています"
+    },
+    "menu": {
+        "tagline": "あなたの種子バンク",
+        "inventory": "在庫",
+        "market": "マーケット",
+        "profile": "プロフィール",
+        "chat": "チャット",
+        "wishlist": "お気に入り",
+        "following": "フォロー中",
+        "calendar": "カレンダー",
+        "settings": "設定"
+    },
+    "settings": {
+        "language": "言語",
+        "systemLanguage": "システムの言語",
+        "langEs": "Español",
+        "langEn": "English",
+        "langPt": "Português",
+        "langAst": "Asturianu",
+        "langFr": "Français",
+        "langDe": "Deutsch",
+        "langJa": "日本語",
+        "about": "このアプリについて",
+        "aboutText": "在来種のためのローカルファースト・暗号化在庫管理。AGPL-3.0。",
+        "aboutOpen": "Tane について"
+    },
+    "home": {
+        "tagline": "地域の種を分かち合い、育てよう",
+        "openMarket": "マーケット",
+        "openMarketSubtitle": "近くの種を見つけて分かち合う",
+        "yourInventory": "あなたの在庫",
+        "yourInventorySubtitle": "種を管理する"
+    }
+}