Hide floating DevEx Sessions selector while WG sessions are paused

[danbaruka]

Summary

The floating DevEx Sessions / "Select a DevEx Session" control still appears site-wide, but weekly DevEx WG sessions are currently paused and are not running. Visitors are prompted to join meetings that are not happening.

Expected

• Hide the floating DevEx Sessions button and panel while sessions are paused
• Keep the component in the codebase so it can be re-enabled quickly when the series resumes
• Adjust Jump-to-Top positioning so it is not offset for a hidden button

Acceptance criteria

• Floating "DevEx Sessions" / "Select a DevEx Session" UI is not rendered in production
• Re-enable path is a single flag (DEVEX_SESSIONS_ENABLED)
• Jump-to-Top remains usable on mobile/desktop
• cd website && npm run build succeeds

Notes

Related historical issue: #232 (session widget wrong link). This change is about pausing the widget entirely, not fixing a link.