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/widgets/frontend.css

@@ -64,7 +64,6 @@
 .zib-widget-wrap { padding:var(--aksrt-widget-space-top,0) var(--aksrt-widget-space-right,0) var(--aksrt-widget-space-bottom,0) var(--aksrt-widget-space-left,0); background-color:var(--aksrt-widget-bg-light,transparent); background-image:var(--aksrt-widget-bg-image-light,none); background-position:center; background-size:cover; }
 .dark-theme .zib-widget-wrap, [data-theme="dark"] .zib-widget-wrap { background-color:var(--aksrt-widget-bg-dark,var(--aksrt-widget-bg-light,transparent)); background-image:var(--aksrt-widget-bg-image-dark,var(--aksrt-widget-bg-image-light,none)); }
 .zib-widget-wrap .widget-container { position:relative; }
-.aksrt-user-widget-wrap > .widget-container > div > .widget-content { padding:0; }
 .aksrt-user-widget-wrap :where(a, button, input, select, textarea, [tabindex]):focus-visible { outline:3px solid var(--focus-color,#ff5473) !important; outline-offset:3px; }
 .dark-theme .aksrt-user-widget-wrap :where(a, button, input, select, textarea, [tabindex]):focus-visible, [data-theme="dark"] .aksrt-user-widget-wrap :where(a, button, input, select, textarea, [tabindex]):focus-visible { outline-color:#ffd166 !important; }