Add same-origin proxy mode for Cypress compatibility

[ctrlaltdylan]

Summary

• Adds --proxy CLI flag that mounts http-proxy-middleware at /__proxy/ to reverse-proxy the app through mock-bridge
• Makes the iframe same-origin, enabling Cypress to interact with embedded app content (fixes Feature request: same-origin / proxy mode for Cypress compatibility #17)
• Includes WebSocket proxying for HMR support
• Fully backwards compatible — no behavior change without --proxy

Usage

npx mock-bridge http://localhost:3000 --proxy

Or test this branch directly:

npx github:ctrlaltdylan/mock-bridge#http-proxy-middleware http://localhost:3000 --proxy

Then in Cypress:

cy.visit('http://localhost:3080');
cy.get('iframe').its('0.contentDocument.body')
  .should('not.be.empty')
  .within(() => {
    cy.contains('Your App Content');
  });

Test plan

• Start with --proxy flag, confirm app loads via /__proxy/ (same-origin iframe)
• Confirm HMR/WebSocket still works through the proxy
• Confirm session tokens and mock App Bridge still function
• Confirm Cypress can access iframe content
• Start without --proxy, confirm no behavior change

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