Add i18n support (multi-language)

[2214962083]

Description

Add internationalization support to the dashboard and landing page.

Priority Languages

1. English (current, default)
2. Chinese (简体中文)
3. Japanese (日本語)
4. Spanish (Español)

Details

• Extract all user-facing strings to translation files
• Auto-detect browser language
• Allow manual language switching
• Landing page and dashboard should both support i18n

Why

CronPulse is built by developers who speak Chinese. Supporting more languages expands the addressable market significantly.