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/class-activator.php

@@ -169,6 +169,9 @@ class PRAutoBlogger_Activator {
 			'prautoblogger_max_word_count'        => 2000,
 			'prautoblogger_topic_exclusions'      => '[]',
 			'prautoblogger_enabled_sources'       => '["reddit"]',
+			'prautoblogger_article_font_family'   => 'default',
+			'prautoblogger_article_font_size'     => 0,
+			'prautoblogger_table_borders'         => '1',
 			'prautoblogger_schedule_time'         => '03:00',
 			'prautoblogger_log_level'             => 'info',
 		];