Support subpath deployment

[berlysia]

fixes #243 partially

The problem occurs because the .js file references in each HTML are written as root-relative paths. If the deployment destination is a subpath (e.g., GitHub Pages), the subpath segment must be included when referencing root-relative paths.

When deploying to https://example.com/path/to/some/dir/, the following mismatch arises, causing the .js file to fail to load:

Reference in the output HTML	Actual file path = Correct reference
/static/client-${hash}.js	/path/to/some/dir/static/client-${hash}.js

This PR enables deployment to subpaths such as GitHub Pages. See https://vite.dev/config/shared-options#base https://vite.dev/guide/env-and-mode#env-variables