web: fix flash of unstyled content during load

Add background color to #main in loader styles to prevent white flash
while app CSS loads after WASM initialization.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

gui/public/loader.js

@@ -30,6 +30,9 @@
     '}' +
     '@keyframes wasm-spin {' +
       'to { transform: rotate(360deg); }' +
+    '}' +
+    '#main {' +
+      'background-color: oklch(0.15 0.01 338.64);' +
     '}';
   document.head.appendChild(style);