Avoid duplicate sitemap roots on subpath deployments - #126

Closes #121

The sitemap integration joins an empty home-page path to base verbatim, but adds a trailing slash for the root route in directory builds. With base set to /my-project, those become two different sitemap entries.

This normalizes base only in the config passed to the sitemap integration. The app's base stays unchanged, and file output and trailingSlash: never retain their existing behavior.

Validation:

• Reproduced the duplicate in generated sitemap XML before the fix
• 10 sitemap and hidden-version tests passed
• Framework build and typecheck passed
• Starter build passed, with the existing Pagefind spawn warning
• ESLint reported no source errors; the test file is outside its configured scope
• templates:check could not spawn pnpm on Windows (ENOENT)
• Stopped the broader test run after failures in CLI path separators, type coverage, and image-reference tests outside this change

Keeping this in draft for CI validation. Prepared with an AI coding assistant