bingo: serve real favicon files

The favicon was an emoji in a data: URI and /favicon.ico was a 404.
Google only shows a site icon in results when the favicon has a
crawlable URL, and the emoji looked different on every platform.

Add an SVG (a 5x5 card with a crossed-out diagonal), a 16/32/48 ICO
and a 180px apple-touch-icon under assets/bingo/, served by explicit
routes because public/ belongs to asxp.io.

Assisted-by: Claude:opus-5

views/bingo/layout.erb

@@ -34,7 +34,9 @@
 <meta name="twitter:title" content="<%= t(:seo_title) %>">
 <meta name="twitter:description" content="<%= desc %>">
 <script type="application/ld+json"><%== json_ld(@board, desc) %></script>
-<link rel="icon" href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>🎱</text></svg>">
+<link rel="icon" href="/favicon.ico" sizes="48x48">
+<link rel="icon" href="/favicon.svg" type="image/svg+xml">
+<link rel="apple-touch-icon" href="/apple-touch-icon.png">
 <style>
   :root {
     --bg: #f4f1ea; --card: #fffdf8; --ink: #1d1b18; --muted: #6f6a60; --line: #d9d3c6;