fix(widgets): restore the 20px bottom gap under the quote and search widgets

The reference wraps these two widgets in a middle .theme-box layer
(widget-container > .theme-box > title + card), which is what supplies the
20px bottom margin. Our outer .jio-widget-box-wrap is only a transparent
layout container, so dropping .theme-box left the quote card flush against
the following module.

- search/quote: emit the reference .theme-box middle layer; the card itself
  stays margin-free like the reference, so the gap is exactly 20px
- keep the generic .widget-content branch for user_card/author_card and the
  other reference widgets untouched (the string is contract-tested)
- contract test now pins the .theme-box layer and asserts the quote card
  carries no margin class of its own

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/widget-system/search-quote-card-contract.test.cjs

@@ -18,16 +18,17 @@ test('搜索框与一言小工具只渲染参考主题的一张卡片', () => {
   assert.doesNotMatch(rendered.search, /jio-widget-shell/);
   assert.doesNotMatch(rendered.quote, /jio-widget-shell/);
 
-  // 搜索框：widget-container > 标题 + 单张 .widget-search 卡片，且复用共享搜索组件。
+  // 搜索框：widget-container > .theme-box（承载 20px 底距）+ 标题 + 单张卡片。
   assert.equal((rendered.search.match(/class="jio-panel/g) || []).length, 1);
-  assert.match(rendered.search, /<div class="widget-container"><div><div class="box-body notop"><div class="title-theme">/);
+  assert.match(rendered.search, /<div class="widget-container"><div class="theme-box"><div class="box-body notop"><div class="title-theme">/);
   assert.match(rendered.search, /class="jio-panel widget-search"><div class="search-input">/);
   assert.equal((rendered.search.match(/class="search-input"/g) || []).length, 1);
 
-  // 一言：widget-container > 标题 + 单张 .yiyan-box 卡片（点击根元素即卡片）。
+  // 一言：同样的 .theme-box 间距层，卡片自身不带底部边距。
   assert.equal((rendered.quote.match(/class="yiyan-box/g) || []).length, 1);
-  assert.match(rendered.quote, /<div class="widget-container"><div><div class="box-body notop"><div class="title-theme">/);
+  assert.match(rendered.quote, /<div class="widget-container"><div class="theme-box"><div class="box-body notop"><div class="title-theme">/);
   assert.match(rendered.quote, /class="yiyan-box main-bg text-center box-body radius8 main-shadow jio-yiyan" data-jio-yiyan>/);
+  assert.doesNotMatch(rendered.quote, /class="yiyan-box[^"]*theme-box/);
   assert.match(rendered.quote, /<div class="yiyan"><\/div>/);
 });