feat: group creation success screen with invite code sharing

[wumibals]

Summary

Updates the group creation success screen to show a toast on copy, use replace navigation so the screen cannot be navigated back to, and display the invite code prominently.

Changes

• Show toast feedback when invite code is copied
• Use router.replace for 'View My Group' navigation (screen cannot be navigated back to)
• Display invite code in large copyable text box
• Share sheet opens via Share.share

Acceptance Criteria

• Invite code is displayed and copyable
• Share sheet opens when 'Share' is tapped
• 'View My Group' navigates to group detail
• Screen cannot be navigated back to (replace navigation)

closes #266

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