Authorization Feedback & Onboarding Pop-ups

**Description:**
When a user logs in via Auth0, it is not clear enough whether they are authorized. First-time users should also see onboarding pop-ups.

**Tasks:**
- [ ] Detect `?code=` and display a **success message** (“✅ You are authorized to use the RERUM API”).
- [ ] Detect `?error=` and display a **clear error message** with details.
- [ ] Update status checker UI with green check (valid) and red cross (invalid).
- [ ] Add **first-time login pop-up**:
- “Welcome! You are now authorized to use the RERUM API. Your tokens will appear below.”

**Acceptance Criteria:**
- User immediately sees whether they are authorized.
- Messages are styled clearly and visible without scrolling.
- First-time login shows a welcome pop-up with guidance.