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

assets/css/core/tokens.css

@@ -138,7 +138,6 @@
   /* ===== Card density — 对齐 legacy .box-body { padding: 15px } ===== */
   --card-padding:       15px;
   --card-padding-sm:    10px;
-  --card-padding-lg:    20px;
   --card-gap:           16px;
   --card-gap-lg:        20px;