fix(dev): use SPA fallback as index.html when public/index.html exists

[Cypher-Aura-19]

What?

In SPA mode (ssr: false without prerendering /), always promote the generated SPA fallback to index.html, even when Vite has already copied a public/index.html into the client build directory.

Why?

Since the v8 Environment API migration, the SPA shell is first generated as __spa-fallback.html and later renamed to index.html.

When an application contains public/index.html, Vite copies that file into the output directory first. The existing finalize logic sees that index.html already exists and skips the rename, leaving the actual generated SPA shell in __spa-fallback.html.

Static hosts then serve the copied public/index.html instead of the React Router SPA shell.

This restores the previous SPA behavior while preserving the existing behavior when / itself is prerendered.

Testing

• Added a regression test for ssr: false with public/index.html
• Verified the generated index.html contains the complete SPA document and hydrates correctly
• Verified the regression test fails without the production fix
• vite-spa-mode: 46/46 passed
• vite-prerender: 37/37 passed
• vite-build: 16/16 passed
• ESLint passed
• Prettier passed
• @react-router/dev typecheck passed
• integration TypeScript check passed
• change-file validation passed

Fixes #15444