fix(widgets): keep widget-content an unstyled structural layer

Reference contract: widget cards carry their own 15px padding and
.widget-content is bare. The added 20px padding inset every widget
card from the content max width on the fixed homepage (the only
bundle loading sidebar.css).

- drop the root padding rule and the unreferenced --flush modifier
- drop the now-redundant user-widget padding:0 workaround
- drop the dead --card-padding-lg token
- lock the contract with negative assertions in the display test

tests/widget-system/page-display-contract.test.cjs

@@ -304,7 +304,10 @@ test('user profile and author widgets render the reference user-card contract',
   // 后加载的 widget bundle 明确保留单层卡片并让封面贴边。
   assert.match(widgetCss, /\.user-card\.zib-widget\s*\{[^}]*padding:\s*0;[^}]*overflow:\s*hidden;/s)
   assert.match(legacyMainCss, /\.user-card\.widget \.user-cover \+ \.card-content\s*\{[^}]*margin-top:\s*-55px;/s)
-  assert.match(widgetCss, /\.aksrt-user-widget-wrap > \.widget-container > div > \.widget-content\s*\{\s*padding:\s*0;/)
+  // 参考主题契约：.widget-content 是无样式结构层，小工具卡片自带内边距；
+  // 任何样式层都不得给 .widget-content 加 padding，否则小工具与内容列宽之间出现左右空隙。
+  assert.doesNotMatch(widgetCss, /\.widget-content[^{,]*\{[^}]*padding/s)
+  assert.doesNotMatch(read('assets/css/frontend/sidebar.css'), /\.widget-content[^{,]*\{[^}]*padding/s)
   assert.doesNotMatch(widgetCss, /user-card__auth-btns/)
   assert.doesNotMatch(widgetCss, /\.aksrt-user-card\s*\{/)
   assert.doesNotMatch(userRenderer, /aksrt-widget-button is-ghost/)