feat(app): add i18n internationalization support

[jwaterwater]

feat(app): add i18n internationalization support

• Add @solid-primitives/i18n dependency
• Create i18n infrastructure (context, hooks, locale files)
• Support 5 languages: English, 简体中文, 日本語, Français, Español
• Auto-detect browser language
• Add language switcher dialog (sidebar settings gear icon)
• Translate home page
• Translate sidebar layout (including command palette)
• Translate session-related pages (session-header, session-new-view)