Consider porting .mjs files to .js with "type": "module"

The v6 migration converted all files from CommonJS to ESM using the .mjs extension. The .mjs extension was a transitional mechanism — the intended long-term approach is "type": "module" in package.json, which makes plain .js files ES modules by default.

This is now the ecosystem convention (Node.js docs, major frameworks, etc.). Keeping .mjs signals an incomplete migration.

The change is mechanical — rename all .mjs → .js, add "type": "module" to package.json, update any import paths — but worth doing for consistency with modern Node.js conventions.

先盤點儲存庫中的 .mjs 檔案並檢查 package.json，然後找出依賴副檔名的匯入路徑。當檔案使用 .js、package.json 宣告 "type": "module"，且所有受影響的匯入都一致更新後，這項變更便完成。