CIAB: Add configurable font styles for partner branding

[valterlorran]

Part of ARC-1388

Proposed Changes

• Extends the partner branding system to support custom font styles on login/signup pages.
• Partners can now specify a fontStyle in their config to override the default heading font (Recoleta).
• Woo branding - Uses the system font stack ($default-font) to match the CIAB dashboard styling.
• Extensible - New font styles can be added by extending the fontStyle type and adding corresponding CSS rules.

Why are these changes being made?

• CIAB aims to provide a consistent branded experience across all touchpoints. The login/signup pages should match the CIAB dashboard font styling for visual consistency. This change allows partners to configure their preferred heading font through the centralized partner branding config.

Testing Instructions

• On an incognito window, open /log-in?from=woo.
• You should see the heading text using the system font (SF Pro on macOS, Segoe UI on Windows) instead of Recoleta.

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)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-168343

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-168343&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-168343&env=a8c-for-agencies

Dashboard live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-168343&env=dashboard

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• agents-manager
• help-center
• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug update/ciab-branding-fonts on your sandbox.