feat: [10-login-page-front] add frontend login page

[MSZM0]

Changes

• A simple login button (later redirecting to usos once possible)
• A page telling the user what data we pull from usos and how we use it
• A profile setup form where a user can enter their nickname if it's their first time logging in

Checklist

• PR is linked to an issue (tab on the right).
• Acceptance criteria (from issue) are met.
• All status checks (CI) are green.
• Tests added / updated.
• Docs updated (if applicable).

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