Complete Japanese localization of UI components

[Copilot]

This PR completes the Japanese localization (日本語化) of the Burner List application by translating all remaining English UI text to Japanese.

What was already localized

The application already had significant Japanese support including:

• Complete Japanese README (README.ja.md)
• Japanese app manifest and metadata
• Basic component headings (フロントバーナー, バックバーナー, キッチンシンク)
• Japanese placeholder text in QuickAdd component

What this PR adds

This PR translates all remaining English UI text across all components:

Core Components:

• FrontCard/BackCard: Placeholders ("最重要なタスクは何ですか？"), buttons ("保存", "キャンセル"), progress labels ("進捗"), and tooltips
• SinkList: Item count display, empty state message ("キッチンシンクにはまだ項目がありません"), and action tooltips
• SettingsSheet: Complete translation of all settings text, session history, keyboard shortcuts, and help text
• RolloverDialog: Session rollover dialog with proper Japanese messaging and option labels
• Main Page: App title, session information, and button labels

Technical Improvements:

• Updated HTML lang attribute from "en" to "ja" for proper language declaration
• Fixed failing Japanese support tests by resolving state pollution issues
• Ensured consistent Japanese terminology throughout the application

Terminology Standards

The translation uses consistent Japanese terminology:

• Front Burner → フロントバーナー
• Back Burner → バックバーナー
• Kitchen Sink → キッチンシンク
• Session → セッション
• Rollover → ロールオーバー

Testing

All tests pass (12/12) including comprehensive Japanese text support tests that verify:

• Japanese text input and storage functionality
• Support for hiragana, katakana, kanji, and emoji
• Mixed Japanese/English text handling

The application now provides a complete native Japanese user experience for Japanese-speaking users.

Fixes #1.

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