add required GDPR consent checkbox to agent registration form

[ivannissimrch]

Description

Adds a required consent checkbox to the last step of the agent/NGO registration form (AccountStep.tsx), with links to the Data Privacy and Agreement pages

Closes #748

Changes

• Gates submission: the form can't be submitted until consent is ticked
• Adds consent to AgentRegistrationData with a default false
• Adds en/de translation strings

Screenshots / Demos

Screencast.from.2026-07-04.09-08-34.mp4

Notes on link routing

The Data Privacy / Agreement links use /${Subpage.DATA_PRIVACY} / /${Subpage.AGREEMENT}
without a lang prefix, since the middleware already injects the locale. Alternative: I could read the locale with useParams and prefix it explicitly, <a href={/${lang}/${Subpage.DATA_PRIVACY}}>…</a>

Both work, but I went with the no-prefix to keep it simple.

Checklist

• WITHIN THE SCOPE OF AN ISSUE; No unnecessary files included
• Tests added/updated
• Documentation updated
• CI passes

[need4deed]

Looks good, we missed on one link to add there the guidlines https://www.need4deed.org/rac-guidelines
Leitlinien

@ivannissimrch can you add it to the pr? between the two other links

[ivannissimrch]

Link for guidelines added.