feat(menu): sync discovery page to Synvya.com on menu publish

[alejandro-runner]

Summary

• Integrate the unified publish flow into the Menu page
• After menu is published to Nostr (both Square and spreadsheet workflows), automatically call fetchAndPublishDiscovery to update the Synvya.com discovery page
• Add multi-step progress indicator showing "1. Publishing to Nostr → 2. Updating discovery page"
• Display Synvya.com-specific errors with a "Contact support@synvya.com" link (pre-populated with error details)
• Nostr publishing succeeds independently; if Synvya.com fails, user sees success for Nostr with separate error for discovery

Test plan

• Verify Square menu publish triggers discovery page update
• Verify Spreadsheet menu publish triggers discovery page update
• Confirm progress indicator shows during multi-step publish
• Test that Nostr publish succeeds even if Synvya.com fails (mock error)
• Verify error message includes contact support link with pre-filled email

Closes #293