fix: return sourcemaps from vite-plugin-sveltekit-remote transform hook

[DJMogens]

Description

The transform hook in plugin_remote (src/exports/vite/index.js) modifies .remote.* files but doesn't return a map property in either code path. This causes:

1. Vite build warnings: [plugin vite-plugin-sveltekit-remote] Sourcemap is likely to be incorrect — once per .remote.* file, for both SSR and client builds.
2. Broken sourcemaps: The resulting .js.map files have sources: [], sourcesContent: [], and semicolon-only mappings. Error monitoring tools (e.g. Sentry) cannot map stack traces back to the original source.

Per the Vite Plugin API docs, transform hooks should return { code, map }.

Closes #15472

Changes

SSR path (~line 727): Uses MagicString (already a dependency) instead of string concatenation, returns { code, map } with a proper sourcemap via ms.generateMap({ hires: true }).

Client path (~line 803): Adds map: { mappings: '' } to signal intentional full module replacement (the generated proxy code doesn't need source mapping back).

Testing

Reproduction repo with build:bug / build:fix scripts: https://github.com/DJMogens/sveltejs-kit-sourcemap-bug

• pnpm format passes
• Added changeset

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🦋 Changeset detected

Latest commit: e7ea2ff

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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