Respect "MODE" to set environment variables from another dotenv file similar to Vite

Describe the bug

Vite uses a process environment variable of MODE to allow a user to declare another .env file like .env.[MODE] and .env.[MODE].local:

https://vite.dev/guide/env-and-mode#env-files

Svelte conflicts with this by only loading .env and .env.local. Instead, Svelte should also load all environment variables from MODE.

Reproduction

• Create a new .env.preview file:

MY_SECRET=1234

• create empty .env and .env.local files

• Run MODE=preview npm run build

If you try to console.log out that secret, it will be undefined, because SvelteKit is not using .env.preview.

Logs

System Info

System:
    OS: macOS 26.6.2
    CPU: (10) arm64 Apple M5
    Memory: 3.02 GB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.16.0 - /Users/zjh/.volta/tools/image/node/24.16.0/bin/node
    Yarn: 4.18.0 - /Users/zjh/.volta/tools/image/yarn/4.18.0/bin/yarn
    npm: 11.17.0 - /Users/zjh/.volta/tools/image/npm/11.17.0/bin/npm
  Browsers:
    Brave Browser: 151.1.93.136
    Safari: 26.6.2
  npmPackages:
    @sveltejs/adapter-auto: ^6.0.0 => 6.1.1 
    @sveltejs/adapter-vercel: 5 => 5.10.3 
    @sveltejs/kit: ^2.22.0 => 2.70.3 
    @sveltejs/vite-plugin-svelte: ^6.0.0 => 6.2.4 
    svelte: ^5.5.0 => 5.56.10 
    vite: ^7.0.4 => 7.3.6

Severity

annoyance

Additional Information

No response

No source file or test is named. Start from the npm run build entry point and trace how .env and .env.local are loaded, comparing that behavior with Vite's MODE-based env-file rules. Done means MODE=preview loads .env.preview and .env.preview.local so MY_SECRET is available.