Fix #1431 browser crypto boundary regression

[BryanHarrisScripts]

Summary

Hotfix the real-machine regression found immediately after #1431 merged.

The new Foundations BUZZ Story Test client imported the Story Bridge adapter, which imported the Story Bridge core, which imported the Nostr signature verifier and node:crypto. Vite externalized node:crypto for browser compatibility and the PlotPickle login/runtime crashed before the user could reach the live test.

This fix:

• removes Story Bridge request preparation from the client component;
• adds a bounded localhost-only prepare action to the existing Story Bridge server gateway;
• keeps Nostr/signature verification and Story Bridge request preparation server-side;
• preserves the exact Add one-click BUZZ Story Bridge live proof #1431 live proof sequence and no-canon-write contract;
• adds a regression guard that fails if the client live-test component references prepareStoryBridgeRequest, nostr-event-verification, or node:crypto again.

Relates to #1422 and repairs #1431. Do not close #1422 until the real Afterglow/Tamsin live test passes on Windows.