fix MPP-4093: ensure data-has-premium always has an explicit string value

[groovecoder]

This PR fixes #MPP-4093.

Our new createElement call does not implicitly convert booleans. createElement treats true as a boolean attribute, which results in <firefox-private-relay-addon-data data-has-premium ...>. In the add-on code, we don't handle the attribute as a boolean - we treat it as a string, so the empty attribute value returns as falsy, which makes the add-on think the user is not premium when they really are.

How to test:

With dev server

1. Push this branch to the dev server
2. Download firefox_relay-2.8.1-dev-server.zip from https://github.com/mozilla/fx-private-relay-add-on/releases/tag/2.8.1
3. In a new Firefox profile, go to about:debugging and install the firefox_relay-2.8.1-dev-server.zip add-on as described here.
   • The browser should open to the relay add-on "first run" page
4. Click the sign up/in button
   • You should be sent to the sign up/in page at https://dev.fxprivaterelay.nonprod.cloudops.mozgcp.net/accounts/profile/
5. Sign up/in to a premium account
   • Either sign up/in to an account with your @mozilla.com address to get Premium, or go thru the purchase flow on a new account
6. With the add-on, generate 6 masks
   • Expected result: you CAN create more than 5 masks!

Locally

In this repo:

1. Check out this branch which includes React 19 and the fix
2. cd frontend && npm run build
3. python manage.py collectstatic
4. python manage.py runserver

In the add-on repo:

1. Check out 2.8.1 tag
2. web-ext run -s src/ -f /Applications/Firefox.app/Contents/MacOS/firefox
   • The browser should open to the relay add-on "first run" page
3. Click the sign up/in button
   • You should be sent to the sign up/in page at 127.0.0.1:8000/accounts/profile
4. Sign into an account that has premium (note: any @mozilla.com account will have premium)
5. With the add-on, generate 6 masks
   • Expected result: you CAN create more than 5 masks!

• l10n changes have been submitted to the l10n repository, if any.
• I've added a unit test to test for potential regressions of this bug.
• I've added or updated relevant docs in the docs/ directory.
• All UI revisions follow the coding standards, and use Protocol / Nebula colors where applicable (see /frontend/src/styles/colors.scss).
• Commits in this PR are minimal and have descriptive commit messages.

[vpremamozilla]

i tested on the dev server. works as intended, see attached video
https://github.com/user-attachments/assets/9dfa92d0-2fd9-4457-8554-d1ccfda721ea