Give the page background a casino felt texture

Replace the flat radial green with a layered body background: a soft
overhead light and an edge vignette over a fine grain tiled across the
green gradient.

The grain is a tiny seamless SVG tile (feTurbulence fractal noise mapped
to semi-transparent black speckles, stitchTiles so it repeats without
seams) committed under web/assets, so it rides the existing asset
pipeline and works offline; the palette and panel/card colors are
unchanged.

web/theme.css

@@ -40,11 +40,27 @@
 body {
     min-height: 100vh;
     background-color: var(--felt-dark);
-    background-image: radial-gradient(
-        ellipse at 50% -10%,
-        var(--felt) 0%,
-        var(--felt-dark) 65%
-    );
+    /* Casino-table felt: a fine grain texture tiled over the green
+     * gradient, lit from above and darkened towards the edges. The first
+     * layer is the topmost one. */
+    background-image:
+        radial-gradient(
+            ellipse at 50% 45%,
+            transparent 55%,
+            rgba(0, 0, 0, 0.38) 100%
+        ),
+        radial-gradient(
+            ellipse at 50% -10%,
+            rgba(255, 255, 255, 0.07),
+            transparent 60%
+        ),
+        url("/assets/felt.svg"),
+        radial-gradient(
+            ellipse at 50% -10%,
+            var(--felt) 0%,
+            var(--felt-dark) 65%
+        );
+    background-repeat: no-repeat, no-repeat, repeat, no-repeat;
     background-attachment: fixed;
     color: var(--text);
 }