build: refresh runtime assets before builds

[jiezhengj]

Splits the runtime-asset build change out of #80 for focused review (4/4).

Summary

• run the existing copy-assets script before a standard production build
• run the same step before both Web and Electron static exports
• prevent packaged FFmpeg core and ONNX Runtime WASM files from being missing or stale when postinstall was skipped or an existing checkout is reused

Scope

This changes only three package scripts. Generated assets, speech models, dist output, and installers are not committed.

Validation

• npm run build
• confirmed copy-assets refreshed FFmpeg core and ONNX Runtime WASM before the successful production build

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