feat: interactive model picker with searchable popup and pricing

Replace plain text inputs for analysis/writing/editor model fields
with a rich model picker that fetches the OpenRouter model registry
and displays a searchable popup with model name, provider, input/output
pricing per 1M tokens, and context window. Also extracts source status
and model field renderers into dedicated field classes to keep
class-admin-page.php under the 300-line cap.

New files:
- assets/js/model-picker.js — popup UI with search, filtering, caching
- assets/css/model-picker.css — popup styles matching admin design system
- includes/admin/fields/class-open-router-model-field.php — server render
- includes/admin/fields/class-source-status-field.php — extracted field

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/class-autoloader.php

@@ -22,6 +22,7 @@ class PRAutoBlogger_Autoloader {
 	private static array $directories = [
 		'',           // includes/ root
 		'admin/',
+		'admin/fields/',
 		'core/',
 		'providers/',
 		'models/',