fix(cli): prevent duplicate SSG output on rebuild

[sanmaxdev]

Summary

• Run SSG builds in client-first sequential order so index.html is refreshed before prerendering starts.
• Keep the existing parallel path for non-SSG builds.
• Correct the --force-sequential help text to match the actual client-first order.

Root cause

The server build could finish first during a parallel fullstack build and start SSG against the previous client output. On repeated builds, that meant prerendering could use already-generated SSG HTML as its template and inject the page again.

Requiring sequential order for SSG makes the client rewrite the clean HTML template before the server starts prerendering.

Verification

• cargo check -p dioxus-cli --tests
• Regression coverage for parallel, explicitly sequential, and SSG build-order selection
• git diff --check

Fixes #5608

[ealmloff]

Thanks! We could still do builds in parallel as long as we keep the ssg rendering after web finishes its index.html update, but this is a fine solution for now