Add Japanese localization for WebUI, reports, and think/diff CLI

[melonattacker]

This pull request introduces comprehensive internationalization (i18n) support for both the CLI and report outputs, adding Japanese language support alongside English. It also adds a Japanese README and improves some English README phrasing. The main changes are grouped below.

Internationalization for CLI output:

• The CLI now supports both English and Japanese output. All user-facing CLI messages (step headers, summaries, metrics, threat previews, etc.) are routed through a translation dictionary and locale normalization function, with the ability to set the locale globally or per instance. (src/threat_thinker/cliui.py) [1] [2] [3] [4] [5] [6] [7]

Internationalization for report/export output:

• All report and diff output strings are now localized, with translation dictionaries and helper functions for both English and Japanese. The appropriate disclaimer is also shown based on language. (src/threat_thinker/exporters.py)

Documentation updates:

• Added a comprehensive Japanese README (README.ja.md) covering features, usage, and installation.
• Updated the English README with improved wording and a prominent language switch link.

Summary of most important changes:

Internationalization support:

• All CLI messages and report outputs are now localized, supporting both English and Japanese, with locale selection and translation logic added. [1] [2]

Documentation:

• Added a detailed Japanese README (README.ja.md) and improved the English README with a language switch and clearer description. [1] [2]

These changes significantly improve the accessibility and usability of Threat Thinker for Japanese-speaking users and lay the groundwork for future language support.