Fix Google Calendar subscription flow

[msaroufim]

Summary

• replace the unreliable Google Calendar ?cid= external-feed shortcut
• copy the public lecture calendar URL to the clipboard
• open Google Calendar’s supported “From URL” settings screen
• provide clear paste instructions and a fallback when clipboard access fails

Root cause

The deployed .ics feed is valid and publicly reachable, but Google rejects the undocumented ?cid=<external feed> subscription shortcut with “Unable to add calendar. Check the URL.” External iCalendar subscriptions are supported through Google Calendar’s “From URL” workflow instead.

Impact

Users can subscribe once to the GPU MODE lecture feed and continue receiving new or updated Discord lectures automatically, without the broken error path.

Validation

• parsed the deployed feed successfully with Python icalendar (3 VEVENTs)
• npm run lint
• npm run build