Feat/implement basic credits wallet frontend

[jc99k]

Description

• Shows credits wallet context in the header and project dashboard when billing is enabled.
• Makes the billing integration opt-in through REACT_APP_BILLING_APP instead of hardcoding a billing app slug.
• Documents the web env var and passes it through local web builds only when configured.

Checklist before requesting a review

• I have performed a self-review of my code.
• My code follows the style guidelines of this project.
• I have made corresponding changes to the documentation.
• New and existing tests pass locally with my changes.
• If this change is a core feature, I have added thorough tests.
• If this change affects or depends on the behavior of other estela repositories, I have created pull requests with the relevant changes in the affected repositories. Please, refer to our official documentation.
• I understand that my pull request may be closed if it becomes obvious or I did not perform all of the steps above.

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[jc99k]

Shows current wallet balance in top right section.

• Balance when user is Project Owner

- Balance when user belongs to Project but it's not Owner (shows user's balance, but project will not be billed against it)