feat(phone-image): add cover_text migration and rewrite CSS to xiaohongshu classic style

T1: Add cover_text text field to post table for phone image layout cover text

T2: Rewrite phone-image-templates.css - remove tpl-magazine/tpl-mixed, merge tpl-minimal into base styles, add content-flow/break-inserter/page-break-marker/page-alignment-toggle styles

database/migrations/20260502000000_add_cover_text_to_post.php

@@ -0,0 +1,14 @@
+<?php
+
+use think\migration\Migrator;
+use think\migration\db\Column;
+
+class AddCoverTextToPost extends Migrator
+{
+    public function change()
+    {
+        $table = $this->table('post');
+        $table->addColumn(Column::make('cover_text', 'text')->setComment('封面文案，用于手机图片排版封面展示')->setNull(true))
+              ->update();
+    }
+}