feat: Migrate to Google Merchant API and remove Composer dependencies

- Fix ISE 500 error on plugin activation:
  - Defer wc_get_logger() call in Logger class (lazy initialization)
  - Move plugin init to plugins_loaded hook with priority 20
  - Remove invalid Google_Service_ShoppingContent_Reports instantiation

- Migrate from deprecated Content API to new Merchant API:
  - Rewrite Google API class with direct REST calls
  - Implement JWT authentication using PHP OpenSSL
  - Use WordPress wp_remote_* functions for HTTP requests
  - Support Price Competitiveness reports for competitor pricing

- Remove all Composer dependencies:
  - Delete vendor/ directory (~50 packages)
  - Delete composer.json and composer.lock
  - Remove setup scripts and related documentation

- Plugin is now fully self-contained with no external dependencies

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

includes/class-informatiq-sp-logger.php

@@ -40,8 +40,19 @@ class Informatiq_SP_Logger {
 	 */
 	public function __construct() {
 		global $wpdb;
-		$this->wc_logger   = wc_get_logger();
-		$this->table_name  = $wpdb->prefix . 'informatiq_sp_logs';
+		$this->table_name = $wpdb->prefix . 'informatiq_sp_logs';
+	}
+
+	/**
+	 * Get WC Logger instance (lazy initialization).
+	 *
+	 * @return WC_Logger|null
+	 */
+	private function get_wc_logger() {
+		if ( null === $this->wc_logger && function_exists( 'wc_get_logger' ) ) {
+			$this->wc_logger = wc_get_logger();
+		}
+		return $this->wc_logger;
 	}
 
 	/**
@@ -77,7 +88,10 @@ class Informatiq_SP_Logger {
 	 * @param array  $context Additional context.
 	 */
 	public function info( $message, $context = array() ) {
-		$this->wc_logger->info( $message, array_merge( array( 'source' => $this->source ), $context ) );
+		$logger = $this->get_wc_logger();
+		if ( $logger ) {
+			$logger->info( $message, array_merge( array( 'source' => $this->source ), $context ) );
+		}
 	}
 
 	/**
@@ -87,7 +101,10 @@ class Informatiq_SP_Logger {
 	 * @param array  $context Additional context.
 	 */
 	public function error( $message, $context = array() ) {
-		$this->wc_logger->error( $message, array_merge( array( 'source' => $this->source ), $context ) );
+		$logger = $this->get_wc_logger();
+		if ( $logger ) {
+			$logger->error( $message, array_merge( array( 'source' => $this->source ), $context ) );
+		}
 	}
 
 	/**
@@ -97,7 +114,10 @@ class Informatiq_SP_Logger {
 	 * @param array  $context Additional context.
 	 */
 	public function warning( $message, $context = array() ) {
-		$this->wc_logger->warning( $message, array_merge( array( 'source' => $this->source ), $context ) );
+		$logger = $this->get_wc_logger();
+		if ( $logger ) {
+			$logger->warning( $message, array_merge( array( 'source' => $this->source ), $context ) );
+		}
 	}
 
 	/**