Add native Farcaster modal for miniapp install + notification permission

[realproject7]

Summary

Dropcast has logic to trigger the native Farcaster modal that asks users to add the miniapp and allow notifications. This is NOT a custom modal — it's the platform-native prompt provided by the Farcaster SDK. Apply this to PlotLink.

Implementation

• Review dropcast for the SDK call that triggers the native add/notification modal
• Likely uses sdk.actions.addFrame() or similar Farcaster SDK method
• Determine the right trigger point (first visit, after reading a story, etc.)
• Only show on Farcaster platform (use existing usePlatformDetection hook)

Acceptance Criteria

• Native Farcaster modal appears prompting user to add PlotLink miniapp
• Modal includes notification permission request
• Only triggers on Farcaster platform, not web
• Does not re-prompt users who have already added/dismissed