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/admin/class-settings-fields.php

@@ -125,27 +125,30 @@ class PRAutoBlogger_Settings_Fields {
 			[
 				'id'          => 'prautoblogger_analysis_model',
 				'label'       => __( 'Analysis Model', 'prautoblogger' ),
-				'type'        => 'text',
+				'type'        => 'model_select',
 				'section'     => 'prautoblogger_models',
 				'default'     => PRAUTOBLOGGER_DEFAULT_ANALYSIS_MODEL,
+				'capability'  => 'text→text',
 				'description' => __( 'Used for source analysis. Pick a cheap, fast model.', 'prautoblogger' ),
 				'badge'       => __( 'Low cost', 'prautoblogger' ),
 			],
 			[
 				'id'          => 'prautoblogger_writing_model',
 				'label'       => __( 'Writing Model', 'prautoblogger' ),
-				'type'        => 'text',
+				'type'        => 'model_select',
 				'section'     => 'prautoblogger_models',
 				'default'     => PRAUTOBLOGGER_DEFAULT_WRITING_MODEL,
+				'capability'  => 'text→text',
 				'description' => __( 'Used for article generation. Quality matters here.', 'prautoblogger' ),
 				'badge'       => __( 'Quality', 'prautoblogger' ),
 			],
 			[
 				'id'          => 'prautoblogger_editor_model',
 				'label'       => __( 'Editor Model', 'prautoblogger' ),
-				'type'        => 'text',
+				'type'        => 'model_select',
 				'section'     => 'prautoblogger_models',
 				'default'     => PRAUTOBLOGGER_DEFAULT_EDITOR_MODEL,
+				'capability'  => 'text→text',
 				'description' => __( 'Used for the chief editor review pass.', 'prautoblogger' ),
 				'badge'       => __( 'Quality', 'prautoblogger' ),
 			],