release: add electron-builder configuration for distributables

[ubugeeei]

Problem

`npm run build` produces `dist/`, `dist-render/`, `dist-electron/` and `bin//`, but no `.dmg` / `.exe` / `.AppImage`. Real users can't install FrameScript without a packaging pipeline.

Expected

• Add electron-builder (or electron-forge) configuration.
• Generate per-platform installers with the bundled Rust binaries and ffmpeg notices.
• Ship a `npm run dist` script that produces signed/unsigned artifacts locally.

Acceptance

• Local `npm run dist` produces a runnable artifact on the host platform.
• Docs (`docs/docs/production.md`) document the new flow.