Add Display settings: article font, font size, table borders

New "Display" admin section with three settings:
- Article Font (7 choices + theme default, Google Fonts loaded on demand)
- Article Font Size (px, 0 = theme default)
- Table Borders (toggle, on by default — adds borders, padding, striping)

Inline CSS injected via wp_head only on singular posts with
_prautoblogger_generated meta — zero impact on other pages.

Extracted Image_Model_Registry to keep settings-fields-extended under
300 lines.

Bump 0.6.3 → 0.7.0.

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

includes/admin/class-settings-fields.php

@@ -60,6 +60,11 @@ class PRAutoBlogger_Settings_Fields {
 				'icon'        => 'dashicons-chart-area',
 				'description' => __( 'Connect Google Analytics 4 for post performance scoring.', 'prautoblogger' ),
 			],
+			'prautoblogger_display' => [
+				'title'       => __( 'Display', 'prautoblogger' ),
+				'icon'        => 'dashicons-editor-textcolor',
+				'description' => __( 'Control how generated articles look on the frontend — fonts, sizes, and table styling.', 'prautoblogger' ),
+			],
 			'prautoblogger_images' => [
 				'title'       => __( 'Images', 'prautoblogger' ),
 				'icon'        => 'dashicons-format-image',