feat(i18n): add Japanese locale, standardize language selector, and bump to v1.2.1

[gzz727007]

🦭 Pull Request Description

Summary of Changes

1. Full Japanese Localization (ja):

   • Added complete 1505-key parity Japanese dictionary src/locales/ja.json, covering all 14 core tools, settings, modals, and pipeline workflows with professional PDF domain terminology (PDF 結合, PDF 分割, 墨消し（黒塗り）, 署名・電子印鑑, 自動化パイプライン, ローカル保管庫, etc.).
   • Integrated Japanese auto-detection (navigator.language.startsWith('ja')) into src/i18n.js.
   • Verified 100% key parity with master English dictionary via tests/i18n.test.js.

2. Language Selector Standardization & Neutral Endonyms:

   • Removed nation-state flag emojis from both desktop Navbar.vue and MobileToolsDrawer.vue to avoid geopolitical sensitivity (e.g. flag representation for Traditional/Simplified Chinese, multi-national languages) and resolve Windows Segoe UI Emoji fallback issues.
   • Standardized options using native endonyms with English first and Simplified Chinese last:
     1. English
     2. Deutsch
     3. Español
     4. Français
     5. 日本語
     6. 简体中文
   • Added desktop Lucide Globe icon indicator for cleaner visual hierarchy.

3. Version Bump to v1.2.1:

   • Bumped version to 1.2.1 across package.json, package-lock.json, and src/config/siteConfig.js.

4. Documentation & GitHub Community Templates Audit:

   • Updated README.md and README.zh-CN.md with KeepAlive draft persistence feature notes, standardized language order, and updated test suite metrics (36 suites, 228 passing tests).
   • Updated .github/CONTRIBUTING.md to reflect full 6-language support and correct locale code conventions.
   • Updated .github/ISSUE_TEMPLATE/bug_report.yml tool dropdown to include PageNumberTool, RedactTool, and ShareReceiveTool.
   • Updated scripts/run_all_tests.js and scripts/e2e_i18n_test.js to include Japanese in multi-language test suites.

Type of Change

• 🐛 Bug fix (non-breaking change fixing an issue)
• ✨ New feature (non-breaking change adding functionality)
• ⚡ Performance optimization (rendering, memory, or bundle size)
• 🎨 UI / UX refinement
• 🌍 Internationalization / Translations (new or updated language)
• 📝 Documentation update (README, Guides, Comments)
• 🧪 Test suite enhancement

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🛡️ Privacy & Quality Checklist

• 100% Client-Side Invariant: All code runs strictly in the browser RAM / WebAssembly. No user documents or telemetry data are ever sent over the network.
• Automated Tests: I have verified that all existing tests pass (npm test -> 36 suites, 228 tests passing 100%).
• New Tests Added: Added parity test for ja.json against en.json in tests/i18n.test.js.
• Responsive & Cross-Browser: Verified that UI works smoothly across desktop and mobile screens.
• Self-Review: I have performed a self-review of my own code and removed unnecessary console.log statements.

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Related Issues

Closes #

[cloudflare-workers-and-pages]

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	pdfseal	d387728	Commit Preview URL Branch Preview URL	Sep 25 2026, 07:15 AM