* fix(frontend): add missing rel="noopener noreferrer" to target="_blank" links
Prevent tabnabbing attacks and referrer leakage by ensuring all
external links with target="_blank" include both noopener and
noreferrer in the rel attribute.
Made-with: Cursor
* style: fix code formatting
Add POST /api/skills/install endpoint to install .skill files from
thread's user-data directory. The endpoint extracts the ZIP archive,
validates SKILL.md frontmatter, and installs to skills/custom/.
Frontend Install buttons now call the API instead of downloading.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>