fix: PHP 8.3 TypeError in articles-list total count (int cast)

number_format() in PHP 8.3 strictly requires int|float; $wpdb->get_var()
returns string|null. Cast $total to (int) at point of assignment in
admin/partials/articles-list.php to fix HTTP 500 on Articles admin page.

Also add PHPUnit test covering the string->int cast contract.

Fixes #31. Version bump 2.5.0 -> 2.5.1.

Agent-Session: fix-articles-500-20260617

tests/unit/ArticlesListTotalCastTest.php

@@ -0,0 +1,32 @@
+<?php
+/**
+ * Test that articles list total count is cast to int.
+ *
+ * @package Peptide_News
+ */
+
+class ArticlesListTotalCastTest extends WP_UnitTestCase {
+    /**
+     * Verify that $wpdb->get_var returning a numeric string does not
+     * cause a TypeError in number_format() (PHP 8.3 strict typing).
+     */
+    public function test_total_cast_prevents_type_error() {
+        $total_as_string = '451'; // what $wpdb->get_var() returns
+        $total = (int) $total_as_string;
+        $this->assertIsInt( $total );
+        $this->assertSame( 451, $total );
+        // This should not throw TypeError on PHP 8.3
+        $formatted = number_format( $total );
+        $this->assertSame( '451', $formatted );
+    }
+
+    /**
+     * Verify null return (missing table) results in 0, not a TypeError.
+     */
+    public function test_null_total_cast_to_zero() {
+        $total = (int) null;
+        $this->assertSame( 0, $total );
+        $formatted = number_format( $total );
+        $this->assertSame( '0', $formatted );
+    }
+}