refactor(2meet-data-optimizer): handle admin form posts before the page renders

- the main page's form handlers move unchanged from render_page() to
  handle_post(), hooked on load-{page}: inside render_page() the admin
  header was already sent, so their wp_safe_redirect() calls could not work
- the setup wizard's step 4 snapshot post moves to handle_snapshot_post() on
  its load hook; a failure redirects with snap_failed=1 for the notice
- Verify: unit 701, integration 577, phpcs, stan; dev36 load hook handles a
  posted reset and redirects before output
- PR: PR-097

admin/class-tmdo-admin.php

@@ -89,13 +89,16 @@ class TMDO_Admin {
 	 * Register the admin menu page under Tools.
 	 */
 	public static function register_menu(): void {
-		add_management_page(
+		$hook_suffix = add_management_page(
 			__( 'WP Data Optimizer', '2meet-data-optimizer' ),
 			__( 'WP Data Optimizer', '2meet-data-optimizer' ),
 			'manage_options',
 			self::MENU_SLUG,
 			array( __CLASS__, 'render_page' )
 		);
+		if ( $hook_suffix ) {
+			add_action( 'load-' . $hook_suffix, array( __CLASS__, 'handle_post' ) );
+		}
 	}
 
 	/**
@@ -326,9 +329,10 @@ class TMDO_Admin {
 	}
 
 	/**
-	 * Render the admin page.
+	 * Handle the page's form posts on load-{page}, before any output, so their
+	 * redirects work (inside render_page() the admin header was already sent).
 	 */
-	public static function render_page(): void {
+	public static function handle_post(): void {
 		if ( ! TMDO_Capability::current_user_can_admin() ) {
 			return;
 		}
@@ -752,6 +756,15 @@ class TMDO_Admin {
 			);
 			exit;
 		}
+	}
+
+	/**
+	 * Render the admin page.
+	 */
+	public static function render_page(): void {
+		if ( ! TMDO_Capability::current_user_can_admin() ) {
+			return;
+		}
 
 		$tabs = array(
 			'dashboard'             => __( '儀表板', '2meet-data-optimizer' ),
@@ -774,7 +787,7 @@ class TMDO_Admin {
 			'hivepress'             => __( 'HivePress 整合', '2meet-data-optimizer' ),
 		);
 
-		$tab = sanitize_key( wp_unslash( $_GET['tab'] ?? 'dashboard' ) );
+		$tab = sanitize_key( wp_unslash( $_GET['tab'] ?? 'dashboard' ) ); // phpcs:ignore WordPress.Security.NonceVerification.Recommended -- tab navigation, no state change.
 		// v2.11.8: legacy tabs removed — redirect bookmarks to current main maintenance entry.
 		if ( in_array( $tab, array( 'sop', 'migration' ), true ) ) {
 			$tab = 'entity-bridge';