Add cross-device usage analytics

[BigBen-7]

Description

Track usage patterns across web and mobile clients.

Requirements

• Web vs mobile usage split
• Feature usage by platform
• Session length comparison
• Conversion by platform
• Platform preference over time

Files to Touch

• analytics/components/charts/CrossDevice.tsx

Complexity: 150 points

[RennyThompson]

i would love to take on this issue.

[grantfox-oss]

🦊 GrantFox — @RennyThompson has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #810)
2. Your PR will be reviewed by the PinSpace-Org maintainers

Good luck! Track your progress on GrantFox.