Restructure shared Astro sources and import aliases (#2737)

shared/components/cards/BodyPlaceholder.astro

@@ -0,0 +1,30 @@
+---
+// Equivalent of cards/body-placeholder.html (ui/svg.html inlined).
+// TODO: the ui/svg.html `border` branch (rect + inline style) is not ported —
+//       body-placeholder.html never sets it (the "border" token in the class
+//       list is just a CSS class).
+interface Props {
+	width?: number;
+	height?: number;
+	ratio?: string;
+	class?: string;
+}
+
+const { width = 400, height = 200, ratio, class: className } = Astro.props;
+---
+
+<div class={`card-body${className ? ` ${className}` : ''}`}>
+	<svg
+		xmlns="http://www.w3.org/2000/svg"
+		class="w-100 border rounded"
+		preserveAspectRatio={ratio ? 'xMidYMid meet' : 'none'}
+		width={width}
+		height={height}
+		viewBox={`0 0 ${width} ${height}`}
+		fill="transparent"
+		stroke="var(--tblr-border-color, #b8cef1)"
+	>
+		<line x1="0" y1="0" x2={width} y2={height}></line>
+		<line x1="0" y1={height} x2={width} y2="0"></line>
+	</svg>
+</div>