No PKCE Implementation in OAuth Authorization Code Flow

## Issue: FINDING-061 - No PKCE Implementation in OAuth Authorization Code Flow

**Labels:** bug, security, priority:high, asvs-level:L2

**ASVS Level(s):** [L2-only]

**Description:**

### Summary
The OAuth flow uses the state parameter for CSRF protection but does not implement Proof Key for Code Exchange (PKCE). ASVS 10.1.2 specifically names PKCE code_verifier as a client-generated secret that should be transaction-specific and session-bound. Without PKCE, the authorization code itself is the sole bearer credential for obtaining tokens, vulnerable to code interception attacks via Referer header leak, browser history, open redirector, malicious browser extensions, or network-level interception.

### Details
**Affected Files and Lines:**
- `src/asfquart/generics.py:48-101` - OAuth flow without PKCE

The flow lacks PKCE protection, making authorization codes vulnerable to interception.

### Recommended Remediation
Implement PKCE (RFC 7636) if the ASF OAuth service supports it:

```python
# On login initiation
code_verifier = secrets.token_urlsafe(64)
code_challenge = base64.urlsafe_b64encode(
hashlib.sha256(code_verifier.encode()).digest()
).rstrip(b'=').decode()

# Store code_verifier in pending_states
pending_states[state]['code_verifier'] = code_verifier

# Include in authorization request
oauth_url = (
f"{OAUTH_URL_AUTHORIZE}?"
f"response_type=code&"
f"client_id={CLIENT_ID}&"
f"redirect_uri={redirect_uri}&"
f"state={state}&"
f"code_challenge={code_challenge}&"
f"code_challenge_method=S256"
)

# On token exchange
code_verifier = state_data['code_verifier']
token_params = {
'code': code,
'code_verifier': code_verifier,
# ... other params
}
```

### Acceptance Criteria
- [ ] PKCE implementation added
- [ ] code_verifier generated
- [ ] code_challenge computed
- [ ] Challenge included in auth request
- [ ] Verifier included in token exchange
- [ ] Unit test verifying the fix

### References
- Source reports: L2:10.1.2.md, L2:10.2.1.md
- Related findings: FINDING-060, FINDING-062
- ASVS sections: 10.1.2, 10.2.1

### Priority
High