fix(main): avoid rewriting dotted paths in --single mode fixes #576

[ahfoysal]

Summary

• Changed SPA fallback rewrite rule to avoid matching dot-containing paths, so static assets are not rewritten to index when using --single.
• Added regression coverage in server tests for:
  • extensionless route (/users/settings) rewrites to index.html.
  • existing dotted asset (/bundle.js) is served directly.
  • missing dotted path (/does-not-exist.js) returns 404.
• Added fixture tests/__fixtures__/server/app/bundle.js for the dotted-asset case.

Testing

• CI=true pnpm lint:style
• CI=true pnpm lint:code
• CI=true pnpm test:tsc
• CI=true pnpm test:unit -- tests/server.test.ts