Add a favicon, home-screen icons and a web manifest

The favicon is a bowl of soup: this household's catalog is half keitto, so it
is at least honest. favicon.svg follows the system theme in the tab strip.

Home-screen icons cannot do the same. They must be opaque and must not change
with the theme, so assets/icon.svg is a separate fixed-colour source, with the
artwork inside the central 80% for Android to mask to any launcher shape. The
same 512 is declared maskable in the manifest.

`make icons` rasterises with rsvg-convert and compresses with
`oxipng -o max --zopfli`, which beat `optipng -o7` at every size — 5860 bytes
against 6109 for the three files. Plain `oxipng -o max` was not an upgrade: it
won the two small icons and lost the 512, ending up larger overall. All output
verified pixel-identical to the rasterizer. The PNGs are committed so the
build needs no rasterizer.

Two things that only fail on a real device, so both are covered by smoke
checks: Go has no mime entry for .webmanifest and served it as octet-stream,
which browsers ignore; and a manifest fetch carries no credentials by default,
so behind Basic auth it needs crossorigin="use-credentials" or it 401s.

mockups/favicon/d-ruisleipa.svg

@@ -0,0 +1,14 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32" role="img" aria-label="Foodster">
+  <!-- Ruisleipä: the Finnish rye loaf, hole in the middle. Nothing else in a
+       tab strip looks like this — which is the point, and also the risk. -->
+  <style>
+    .hole { fill: #ECEDE8; }
+    .loaf { fill: #6B4636; }
+    @media (prefers-color-scheme: dark) {
+      .hole { fill: #121316; }
+      .loaf { fill: #9A6E52; }
+    }
+  </style>
+  <circle class="loaf" cx="16" cy="16" r="14"/>
+  <circle class="hole" cx="16" cy="16" r="3.8"/>
+</svg>