Run frontend integration verification

[FriggemannMichael]

Objective:
Verify backend behavior against the provided vanilla JavaScript frontend.

Acceptance criteria:

• Frontend can register and log in users.
• Token-authenticated pages load without authorization errors.
• Profile editing works with FormData and file upload.
• Offer list filters, pagination, detail pages, order creation, reviews, and base info are checked manually.
• Any frontend/backend mismatch is documented as a follow-up issue.

[FriggemannMichael]

TDD workflow for this project:

• Define the expected behavior before implementing the endpoint or model behavior.
• Write or outline the failing test first.
• Implement only enough code to make that test pass.
• Refactor after the test passes.
• Keep Django check, Ruff, and the relevant tests green from the local virtual environment.