P2: Move `i18n-plugin.ts` from `@openelement/app` to `@openelement/adapter-vite`

[SisyphusZheng]

What to build

packages/app/src/i18n-plugin.ts imports vite and node:* modules. Move it to the adapter layer so app remains runtime-free.

Acceptance criteria

• File moved to packages/adapter-vite/src/.
• All imports updated.
• deno-api:check still passes for app.

Blocked by

None