dx: error overlay — show compile errors in the browser

[justrach]

Problem

When a page has a compile error, the browser shows a blank page or stale content. The developer has to check the terminal to see the error.

Proposed Solution

When `zig build` fails, display the error message directly in the browser via the SSE hot-reload channel:

1. Watcher detects file change → triggers rebuild
2. If build fails, capture stderr
3. Send error message over SSE
4. Client-side script shows a styled error overlay with:
   • File path + line number
   • Error message
   • Code snippet with the error highlighted
   • "Fix and save to retry" hint

Similar to Next.js / Vite error overlay but for Zig compile errors.

Complexity

Low. The SSE channel exists. Just need to capture build stderr and send it as a different event type.