fix(html): move background skin min dimensions to in-flow element

The skin absolutely-positions <background-video> (position: absolute;
inset: 0), so min-* on its :host did not reserve layout space — the
parent skin could still collapse and the video would overflow.

Move the 300x150 floor onto background-video-skin, which is in flow
(display: block; width: 100%; height: 100%), so the min-* actually
reserves space when the parent is unsized.

Refs #1497

Author: spuppo-mux

packages/html/src/define/background/skin.css

@@ -7,7 +7,9 @@ background-video-skin {
   position: relative;
   display: block;
   width: 100%;
+  min-width: 300px;
   height: 100%;
+  min-height: 150px;
   object-fit: var(--media-object-fit);
 }
 

packages/html/src/media/background-video/index.ts

@@ -7,8 +7,6 @@ function getTemplateHTML(attrs: Record<string, string>) {
     <style>
       :host {
         position: relative;
-        min-width: 300px;
-        min-height: 150px;
       }
 
       video {