Website fixes: syntax errors, string escaping, and 404 page

[brainkim]

Summary

• Fix string literal escaping in acorn JSX transform - multi-line strings in JSX attributes were outputting raw newlines causing syntax errors
• Add formatted syntax error messages with code context and pointer (similar to Babel)
• Add 404 page support with catch-all route and static 404.html generation

Syntax Error Example

Before: SyntaxError: Unexpected token (4:22)

After:

SyntaxError: Unexpected token (4:22)

     2 |   const x = 1;
     3 |   return (
>    4 |     <div onclick={foo(}>hello</div>
       |                       ^
     5 |   );
     6 | }

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