Add group invite links for easier onboarding

[vayungodara]

Problem

No easy way to invite people to a group. Users need to be found within the app, which creates friction for new user onboarding.

What to change

• Generate unique, shareable invite links per group
• Add "Copy invite link" button to group settings
• Create a join page at /invite/[code] that handles:
  • Existing users: auto-join the group
  • New users: sign up flow then auto-join
• Add expiration and usage limit options for invite links

Acceptance criteria

• Invite links generated and stored in Supabase
• Copy-to-clipboard button in group settings
• Join flow works for existing and new users
• Links can be expired or revoked by group admin

Label: feature | Priority: medium

[vayungodara]

Already implemented: app/join/[code] route handles group invite links.