build: pin supported Node and package manager versions

[ubugeeei]

Finding

The repo does not declare supported Node/npm versions, while Vite 7 and Electron tooling are version-sensitive.

Evidence

• README only says Requires Node.js.
• package.json lacks engines and packageManager.
• Local audit used Node v24.15.0 and npm 11.12.1.

Acceptance criteria

• Add engines/packageManager or an equivalent version file.
• Align CI setup-node with the declared version.
• Update README prerequisites.

[ubugeeei]

対応済みです。修正は PR #1 / commit 9cf4498 に反映済みです。