#32112221 ARC-1516: refactor /log-in/link with connect-screen primitives

Part of #ARC-1516

Proposed Changes

• Refactor the /log-in/link magic-login route to use connect-screen primitives by replacing legacy inline ToS/button UI with shared ConsentText and ActionButtons components.
• Add partner-branding wiring to magic-login so ?from=woo can drive ARC-1516 branding behavior (partner-specific heading + ToS fallback handling).
• Consolidate partner-branding query argument lookup into a single selector path to reduce redundant subscriptions.
• Harden connect-screen external links by always adding noopener noreferrer for _blank targets, with focused unit tests.
• Fix a magic-login state comparison bug (prevProps.site -> prevState.site) that could trigger unnecessary header updates on rerender.

Why are these changes being made?

• ARC-1516 aims to move /log-in/link toward shared connect-screen building blocks while keeping behavior stable and the review surface small.
• This follows prior ARC login/connect migrations by reusing common primitives, improving consistency, and reducing one-off login page implementations.
• It also ensures partner branding can be applied to this route with the current ?from=woo strategy without introducing broader auth-flow changes.

Testing Instructions

• Run yarn eslint client/login/magic-login/index.jsx client/login/magic-login/request-login-email-form.jsx client/lib/partner-branding.tsx client/components/connect-screen/login-page-wrapper/index.tsx client/components/connect-screen/permissions-list/index.tsx client/components/connect-screen/login-page-wrapper/test/index.tsx client/components/connect-screen/permissions-list/test/index.tsx
• Run yarn test-client client/components/connect-screen/login-page-wrapper/test/index.tsx client/components/connect-screen/permissions-list/test/index.tsx client/login/magic-login/test/build-enter-password-login-parameters.test.js
• Manual checks:
  • Visit /log-in/link and verify the form still submits email links, and the page shows connect-screen styled consent text and action buttons.
  • Branding flow setup:
    • Ensure the ciab/custom-branding feature flag is enabled in your local environment.
    • Visit /log-in/link?from=woo and confirm the initial heading reads Log in to Woo.
    • Confirm Woo-branded ToS copy is rendered (“Just a little reminder…”) and links open with target=_blank plus rel="noopener noreferrer".
  • Branding fallback checks:
    • Visit /log-in/link?from=unknown-partner and confirm default (non-partner) heading + ToS copy are shown.
    • Disable ciab/custom-branding, revisit /log-in/link?from=woo, and confirm it falls back to default heading + ToS copy.
  • Verify top-bar links rendered by connect-screen wrappers still include secure rel attributes when opened in a new tab.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the “[Status] Needs Privacy Updates” label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?