[starter] Internships toast on /internships/ only (cooldown + “not open yet”)

## What
- triggers only for /internships/ or #internships
- separate cooldown via localStorage
- copy includes “Not open yet. Still: PRs welcome. PRs are our short list.”

## Constraints
- Static only
- No trackers
- No external JS deps

## Acceptance criteria
- [ ] Works on GitHub Pages (relative paths)
- [ ] Works on propertools.be (absolute paths or base-aware paths)
- [ ] No console errors
