Restore floating DevEx Sessions button with biweekly Thursday 18:00 UTC call

[danbaruka]

Summary

Re-enable the floating DevEx Sessions button with a single biweekly session now that the WG call schedule is back.

Session details

• Name: Developer Experience WG
• Cadence: Every other Thursday (biweekly)
• Time: 18:00 UTC
• First session: Thursday, 17 September 2026
• Duration of series: 1 year (through 17 September 2027)
• Meet link: https://meet.google.com/npt-phro-wja

Acceptance criteria

• Floating DevEx Sessions control is visible again
• Only one session is listed (no morning/evening split)
• Next occurrence resolves from 2026-09-17T18:00:00Z on a 2-week interval
• Google Calendar add uses the Meet URL and stops after the 1-year end date
• Jump-to-Top positioning accounts for the floating button
• cd website && npm run build succeeds

Notes

Reverses the pause from #287 / #288 by flipping DEVEX_SESSIONS_ENABLED back on and replacing meetup-sessions.json.