Resolves#4684
## Description:
Fixes `getApiBase()` returning `https://undefined` on `localhost` when
`API_DOMAIN` is unset (a plain `npm run dev`).
- `src/client/Api.ts`: use the exact `process.env.API_DOMAIN` form so
Vite's `define` replaces it (the previous `process?.env?.API_DOMAIN`
optional-chaining form wasn't matched, leaving the literal string
`"undefined"`).
- `vite.config.ts`: `JSON.stringify(env.API_DOMAIN ?? "")` so an unset
value becomes a falsy empty string instead of the truthy string
`"undefined"`.
Net: an unset `API_DOMAIN` now falls back to `http://localhost:8787` as
intended.
## Please complete the following:
- [x] I have added screenshots for all UI updates — _no UI changes_
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file — _no user-facing text_
- [x] I have added relevant tests to the test directory — new
`tests/Api.test.ts` reproduces `https://undefined` (RED) and asserts the
`http://localhost:8787` fallback (GREEN); full suite green (2149 tests)
## Please put your Discord username so you can be contacted if a bug or
regression is found:
_⚠️ maintainer to fill_
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