chore: [app-router-migration 27] migrate the "signup" page

[grzpab]

What does this PR do?

• This PR migrates the /signup page to the app directory (which runs under the App Router).
• The migrated page exists under /future/signup and is accessible at all times.
• We support a redirect from the former to the latter with the middleware and environmental variables, as done previously.

Type of change

• [X] Chore (refactoring code, technical debt, workflow improvements)
• [X] New feature (non-breaking change which adds functionality)

How should this be tested?

There should be no changes between the /signup and /future/signup in terms of performance and content (on Lighthouse both pages get near 100 scores).

If the APP_ROUTER_SIGNUP_ENABLED is set to 1 and AB_TEST_BUCKET_PROBABILITY is set to 100, then the future signup page should be rendered in place of the /signup page WITHOUT a visible redirect. We can see it by inspecting the existence of the x-calcom-future-routes-enabled cookie.

Mandatory Tasks

• [X] Make sure you have self-reviewed the code. A decent size PR without self-review might be rejected.

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📦 Next.js Bundle Analysis for @calcom/web

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This PR introduced no changes to the JavaScript bundle! 🙌

[PeerRich]

whats needed here to leave draft @grzpab

[hbjORbj]

@PeerRich Sorry for the delay. We got it ready now. @grzpab will turn it ready for review soon.