add favicon to dashboard

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Co-Authored-By: Claude Opus 4.5 <[e-mail]>

src/server/dashboard.zig

···
         \\  <meta charset="UTF-8">
         \\  <meta name="viewport" content="width=device-width, initial-scale=1.0">
         \\  <title>music atmosphere</title>
+        \\  <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>">
         \\  <style>
         \\    * { box-sizing: border-box; margin: 0; padding: 0; }
         \\    body {