fix(oidc): Add state parameter and validate nonce against login CSRF

The OIDC implicit-flow authorize request carried no state parameter and
the nonce was generated with weak Math.random() and never validated
client- or server-side, enabling login CSRF / forced authentication /
replay (AUTHZ-VULN-05).

Generate state and nonce with the Web Crypto CSPRNG, send state in the
authorize request, and validate both state and the id_token nonce claim
in the callback before storing tokens. Forward the nonce to /validate-
token so the backend also binds the id_token to the login flow.

Fixes the forged-callback fragment attack where a planted id_token with
a mismatched nonce was accepted, stored, and used by the SPA.

tests/test_main.py

@@ -106,6 +106,41 @@ def test_validate_token_invalid(mock_verify_token):
     assert not data["valid"]
     assert data["error"] == "Invalid token"
 
+@patch('main.verify_token')
+def test_validate_token_passes_nonce_to_verify(mock_verify_token):
+    """The endpoint must forward the supplied nonce to verify_token (anti-replay binding)."""
+    mock_verify_token.return_value = {
+        "email": "rene.luria@infomaniak.com",
+        "iss": "https://login.infomaniak.com",
+        "aud": "test-client-id",
+        "nonce": "abc123",
+    }
+
+    response = client.post("/validate-token", json={
+        "id_token": "valid.token.here",
+        "nonce": "abc123"
+    })
+
+    assert response.status_code == 200
+    assert response.json()["valid"]
+    # verify_token must have been called with the nonce as the second positional arg
+    assert mock_verify_token.call_args.args[1] == "abc123"
+
+@patch('main.verify_token')
+def test_validate_token_rejects_nonce_mismatch(mock_verify_token):
+    """A mismatched nonce (forced-auth / replay token) is rejected by the backend."""
+    mock_verify_token.side_effect = ValueError("Nonce mismatch: token is not bound to this login")
+
+    response = client.post("/validate-token", json={
+        "id_token": "forged.token.here",
+        "nonce": "abc123"
+    })
+
+    assert response.status_code == 200
+    data = response.json()
+    assert not data["valid"]
+    assert "Nonce mismatch" in data["error"]
+
 def test_validate_token_empty_request():
     """Test validation with empty token."""
     response = client.post("/validate-token", json={