Use kit.config.paths.base in local development

[metonym]

Describe the problem

Consider the use case of deploying to GitHub Pages, where a base URL must be specified (e.g., the name of the repo).

However, when developing a SvelteKit app locally, the base URL cannot be specified. A "not found" error is shown when attempting to navigate to http://localhost/<base-url>/.

As a result, base must be conditionally applied only when building the app for production.

/** @type {import('@sveltejs/kit').Config} */
const config = {
  kit: {
    adapter: adapter(),
    paths: {
       base: process.env.NODE_ENV === "production" ? "/repo-name" : "",
    },
  },
};

Prior Art

vite uses the base URL when developing locally:

// vite.config.js
import { defineConfig } from "vite";

export default defineConfig({
  base: "/repo-name/",
});

Run vite dev:

vite v2.7.13 dev server running at:

> Local: http://localhost:3000/repo-name/
> Network: use `--host` to expose

ready in 342ms.

Describe the proposed solution

The desired UX would be to avoid conditionally applying a base path:

/** @type {import('@sveltejs/kit').Config} */
const config = {
  kit: {
    adapter: adapter(),
    paths: {
       base: "/repo-name"
    },
  },
};

This would also make it easier to preview the app locally (i.e. svelte-kit preview).

Alternatives considered

No response

Importance

nice to have

Additional Information

No response

[Conduitry]

Sounds like a duplicate of #2958.

[metonym]

Oof. Yeah, this is a dupe.