Serve the BIMI logo, favicon.ico and an apple-touch icon as real files

ultimo.email is the webmail itself, so any path that was not a real file fell
back to the app's index.html: /bimi/ultimo-logo.svg (the BIMI record's logo)
and /favicon.ico both came back as the webmail page. They now ship in
frontend/public, which nginx serves before the SPA fallback. The BIMI SVG
moves there from frontend/icons, so there is one copy.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/index.html

@@ -4,6 +4,7 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <link rel="icon" type="image/svg+xml" href="/ultimo-logo-signet.svg" />
+    <link rel="apple-touch-icon" href="/apple-touch-icon.png" />
     <title>Ultimo Email</title>
   </head>
   <body>