Newsletter: Implement comprehensive settings UI with DataForm

[dsas]

Fixes DOTCOM-15286

Proposed changes:

• Migrate Newsletter settings from old Jetpack React implementation to WordPress DataForm-based UI

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

No changes to data tracking or activity tracking.

Testing instructions:

Prerequisites:

• WordPress installation with Jetpack Newsletter package active
• Block theme installed (to test isSubscriptionSiteEditSupported)

Test Steps:

1. Navigate to the Newsletter Settings page in WordPress admin
2. Verify the header displays with Jetpack logo and "Newsletter Settings" title inline
3. Test the Newsletter section:
   • Toggle the main newsletter subscription setting
   • Verify the toggle switches immediately (auto-save)
   • When enabled, verify "Manage all subscribers" link appears
4. Test the Subscriptions section:
   • Toggle various subscription placement options (post end, pop-up, overlay, floating button, navigation, comments)
   • Verify all controls are toggle switches (not checkboxes)
   • Verify "Save" button appears at bottom of card
   • Verify "Save" button is disabled until you make changes
   • Make changes and click "Save"
   • Verify success snackbar notification appears
5. Test the Paid Newsletter section:
   • Verify "Add Plans" button appears with correct link
6. Test the Email configuration section:
   • Toggle "Featured image in emails" - verify auto-save
   • Select "Full text" or "Excerpt" for email content
   • Toggle "Show author avatar" - verify "Update my Gravatar" link appears when enabled
   • Toggle "Show author display name" and "Add post date"
   • Select reply-to settings (comment/author/no-reply)
   • Verify all "Learn more" links are present and functional
7. Test the Sender name field:
   • Edit the sender name input
   • Verify "Save" button appears inline when changed
   • Click "Save" and verify success snackbar
   • Verify preview updates correctly
8. Test the Newsletter categories section:
   • Toggle "Enable newsletter categories" - verify auto-save
9. Verify all cards have proper styling with no internal border separators
10. Verify all labels are in sentence case (not UPPERCASE)

[github-actions]

Are you an Automattician? The PR will need to be tested on WordPress.com. This comment will be updated with testing instructions as soon the build is complete.

[github-actions]

Thank you for your PR!

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/newsletter/src/class-settings.php	0/80 (0.00%)	0.00%	35 💔

Full summary · PHP report · JS report

_{If appropriate, add one of these labels to override the failing coverage check: Covered by non-unit tests Use to ignore the Code coverage requirement check when E2Es or other non-unit tests cover the code Coverage tests to be added later Use to ignore the Code coverage requirement check when tests will be added in a follow-up PR I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage.}