fix: localize agent skill options

[ziwon]

Description

Localizes the built-in Agent Skills displayed in the agent editor.

The backend returns Skill metadata directly from SKILL.md, so the skill cards could appear in Chinese even when the UI locale is Korean or English. This PR adds display-only localized names and descriptions for known preloaded skills while preserving the original skill identifiers used by selected_skills and backend execution.

It also keeps the Skills menu gated by sandbox availability and defaults the loading state to visible until the backend explicitly reports skills_available: false, avoiding a brief disappearance during dependency loading.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📚 Documentation update
• 🎨 Refactor
• ⚡ Performance improvement
• 🧪 Test
• 🔧 Configuration / Build / CI

Related Issue

N/A

Testing

• Ran npm ci in frontend/
• Ran npm run build-only in frontend/
• Verified the change is limited to display text: checkbox values still use the original Skill name

Checklist

• make fmt && make lint && make test pass locally
• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation (README, docs/, Swagger annotations, etc.)
• Breaking changes are clearly called out in the description above

Screenshots / Recordings

Not attached. This is a small text-only UI localization fix for the Agent Skills selection list.