feat(cli): Add custom single-page fallback file

[ahfoysal]

Closes #855.

Summary

• Adds --single-file to customize SPA fallback target when --single is used.
• Normalizes custom fallback paths so foo.html becomes /foo.html.
• Keeps --single default behavior unchanged at /index.html.
• Updates help text and CLI argument parsing/types.
• Adds a targeted CLI test for parsing the new option.

This follows existing --single rewrite behavior and reuses serve-handler rewrites without schema changes.