Fix the two CI failures: svelte-kit sync and the runtime DLLs

[thorstenalpers]

Summary

Two CI failures that surfaced once the lint step stopped hiding them:

• frontend: npm ci no longer ran svelte-kit sync (the prepare script was dropped from package.json), so .svelte-kit/tsconfig.json was missing and eslint reported every import as an unresolved error type. The script is back.
• rust: scripts/stage-runtime.ps1 looked for the sherpa-onnx DLLs in target/<profile>, where the build script only copies them when it actually runs. With a warm Rust cache it does not. The script now reads them from target/sherpa-onnx-prebuilt, which persists, and rebuilds the crate once if that is empty. The same failure made the Release workflow fail on the merge of Format the data model doc #11, so no release was published.

Verification

Locally, with .svelte-kit regenerated from scratch: npm run lint passes, and the stage script copies all four DLLs into src-tauri/runtime. The cold-cache fallback in the script has not been exercised.

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