feat: Switch to OAuth 2.0 authentication for Google Merchant API

- Replace service account authentication with OAuth 2.0 user flow
- Add "Authorize with Google" button in admin settings
- Handle OAuth callback and token exchange
- Store refresh token for automatic access token renewal
- Add revoke authorization functionality
- Update admin UI to show authorization status
- Update price updater to use new OAuth credentials
- Add CSRF protection with state parameter

This change supports organizations that have disabled service account
key creation via iam.disableServiceAccountKeyCreation policy.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/js/admin.js

@@ -19,6 +19,7 @@
 		bindEvents: function() {
 			$('#informatiq-sp-manual-sync').on('click', this.handleManualSync);
 			$('#informatiq-sp-test-connection').on('click', this.handleTestConnection);
+			$('#informatiq-sp-revoke-auth').on('click', this.handleRevokeAuth);
 		},
 
 		/**
@@ -138,6 +139,46 @@
 					}, 5000);
 				}
 			});
+		},
+
+		/**
+		 * Handle revoke authorization button click
+		 */
+		handleRevokeAuth: function(e) {
+			e.preventDefault();
+
+			var $button = $(this);
+
+			// Confirm action
+			if (!confirm(informatiqSP.strings.revokeConfirm || 'Are you sure you want to revoke Google authorization?')) {
+				return;
+			}
+
+			// Disable button and show loading state
+			$button.prop('disabled', true).text(informatiqSP.strings.revokeInProgress || 'Revoking...');
+
+			// Make AJAX request
+			$.ajax({
+				url: informatiqSP.ajaxUrl,
+				type: 'POST',
+				data: {
+					action: 'informatiq_sp_revoke_auth',
+					nonce: informatiqSP.nonce
+				},
+				success: function(response) {
+					if (response.success) {
+						// Reload page to show updated status
+						location.reload();
+					} else {
+						alert('Error: ' + (response.data.message || 'Unknown error'));
+						$button.prop('disabled', false).text('Revoke Authorization');
+					}
+				},
+				error: function(jqXHR, textStatus, errorThrown) {
+					alert('Error: ' + errorThrown);
+					$button.prop('disabled', false).text('Revoke Authorization');
+				}
+			});
 		}
 	};