Add share to X and copy link buttons alongside Farcaster share

[realproject7]

Summary

Currently the story page only has a single "Share to Farcaster" button. Dropcast has three share options — share to X, share to Farcaster, and copy link. Apply the same structure to PlotLink.

Current behavior

Only one button: "Share to Farcaster"

Expected behavior

Three share buttons matching Dropcast's pattern:

1. Share to X — opens Twitter/X compose with story link + text
2. Share to Farcaster — existing Farcaster share (via SDK composeCast or Warpcast intent)
3. Copy Link — copies story URL to clipboard with toast confirmation

Implementation

• Review dropcast share logic for the component structure and share URL patterns
• Add all three buttons in a row or group below the chapters section

Acceptance Criteria

• Three share buttons visible on story page
• Share to X opens Twitter compose with pre-filled text + story URL
• Share to Farcaster works as current implementation
• Copy Link copies URL and shows confirmation feedback