[Feature Request] Account Portability: Dual-Email Linking and Improved Workspace Onboarding

### Context
I am currently onboarding for the **Hugging Face "AI Agents Course" course**. During the sign-up and certification process, I identified a workflow friction that poses a risk to user identity retention and certification validity.

### The Problem
Currently, the onboarding flow creates ambiguity regarding personal vs. professional identities:

1. **Workspace Friction:** Immediately after signing in (via email), the system prompts users to join a company workspace. This happens even if the user intends to engage in a personal capacity, creating confusion.
2. **Certification Vendor Lock-in:** If a user signs up with a corporate email (`name@company.com`) to join that workspace, their course progress and certifications are tied to that email.
3. **The "Bus Factor" Risk:** If a user leaves that company and loses access to the email, they risk losing access to their certifications and Hugging Face profile history.

### Proposed Solution
I propose aligning the Hugging Face account model closer to GitHub's "Personal Account + Organization Access" model.

#### 1. Allow Multiple Emails per Account
Users should be able to link a **Primary Personal Email** (for recovery/certs) and a **Secondary Work Email** (for SSO/Workspace access) to a single User ID.

#### 2. Decouple Certifications from Email Domains
Certifications should be issued to the unique `user_id` or handle, not the email address. This ensures that even if a work email is removed from the profile, the certification remains attached to the user.

#### 3. Revised Onboarding Logic
Please see the proposed logic flow below:

- **User signs up.**
- **System asks:** "Are you using this account for Personal use or Work?"
- *If Personal:* Default to personal profile, optional "Join Org" later.
- *If Work:* Prompt to link corporate email as a *secondary* alias, keeping the personal email as the recovery root.

### Benefits
* **Long-term Retention:** Users keep their accounts (and certifications) throughout their entire career, not just their tenure at one company.
* **Data Integrity:** Prevents "orphaned" certifications when employees leave organizations.
* **Better UX:** Reduces friction for users taking courses on their own time using work computers, or vice versa.

Thank you for considering this improvement to the learner experience!