refactor(admin): split 1089-line admin class into 7 focused files (#7)

Addresses excessive line count in Peptide_News_Admin (1089 lines)
by extracting responsibilities into single-purpose classes:

- class-pn-admin-assets.php (88 lines): CSS/JS enqueuing
- class-pn-admin-menu.php (98 lines): Menu & submenu registration
- class-pn-admin-field-renderers.php (325 lines): 25+ field renderers (static)
- class-pn-admin-settings.php (289 lines): Settings registration & sanitization
- class-pn-admin-log-viewer.php (155 lines): Plugin log viewer UI & AJAX
- class-pn-admin-settings-page.php (214 lines): Settings page + manual buttons
- class-pn-admin-dashboard-pages.php (109 lines): Dashboard/articles/cost pages
- class-peptide-news-admin.php (143 lines): Thin orchestrator (delegator)

All files under 300 lines. Original public interface preserved exactly
so no consumer code or tests break. Updated ARCHITECTURE.md.

Co-authored-by: peptiderepo <peptiderepo@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

admin/class-pn-admin-menu.php

@@ -0,0 +1,98 @@
+<?php
+declare( strict_types=1 );
+/**
+ * Admin menu registration.
+ *
+ * Registers the main Peptide News admin menu and subpages
+ * (Dashboard, Settings, Articles, Cost Tracking).
+ *
+ * Callbacks are provided externally (via the main Admin class).
+ *
+ * @since 2.5.0
+ * @see Peptide_News_Admin — Main admin orchestrator
+ */
+class Peptide_News_Admin_Menu {
+
+	/** @var callable Dashboard page renderer */
+	private $dashboard_callback;
+
+	/** @var callable Settings page renderer */
+	private $settings_callback;
+
+	/** @var callable Articles page renderer */
+	private $articles_callback;
+
+	/** @var callable Cost dashboard page renderer */
+	private $cost_callback;
+
+	/**
+	 * Constructor.
+	 *
+	 * @param callable $dashboard_callback Renderer for dashboard page.
+	 * @param callable $settings_callback Renderer for settings page.
+	 * @param callable $articles_callback Renderer for articles page.
+	 * @param callable $cost_callback Renderer for cost dashboard page.
+	 */
+	public function __construct(
+		callable $dashboard_callback,
+		callable $settings_callback,
+		callable $articles_callback,
+		callable $cost_callback
+	) {
+		$this->dashboard_callback = $dashboard_callback;
+		$this->settings_callback  = $settings_callback;
+		$this->articles_callback  = $articles_callback;
+		$this->cost_callback      = $cost_callback;
+	}
+
+	/**
+	 * Register all admin menu pages.
+	 *
+	 * Creates the main menu (Analytics Dashboard) and three submenus
+	 * (Settings, Articles, Cost Dashboard).
+	 *
+	 * @return void
+	 */
+	public function add_admin_menu(): void {
+		// Main menu — Analytics Dashboard.
+		add_menu_page(
+			__( 'Peptide News', 'peptide-news' ),
+			__( 'Peptide News', 'peptide-news' ),
+			'manage_options',
+			'peptide-news-dashboard',
+			$this->dashboard_callback,
+			'dashicons-rss',
+			30
+		);
+
+		// Sub-menu — Settings.
+		add_submenu_page(
+			'peptide-news-dashboard',
+			__( 'Settings', 'peptide-news' ),
+			__( 'Settings', 'peptide-news' ),
+			'manage_options',
+			'peptide-news-settings',
+			$this->settings_callback
+		);
+
+		// Sub-menu — Articles.
+		add_submenu_page(
+			'peptide-news-dashboard',
+			__( 'Articles', 'peptide-news' ),
+			__( 'Articles', 'peptide-news' ),
+			'manage_options',
+			'peptide-news-articles',
+			$this->articles_callback
+		);
+
+		// Sub-menu — Cost Dashboard.
+		add_submenu_page(
+			'peptide-news-dashboard',
+			__( 'LLM Costs', 'peptide-news' ),
+			__( 'LLM Costs', 'peptide-news' ),
+			'manage_options',
+			'peptide-news-costs',
+			$this->cost_callback
+		);
+	}
+}