fix(errors): use relative import so main-process bundle resolves

Rollup in vite.main.config.ts doesn't know about the `@/` alias (only the
renderer configs do). After PR 4 made `lib/errors.ts` reachable from main,
its `@/config/env` import broke the main build with:

  [vite]: Rollup failed to resolve import "@/config/env" from "lib/errors.ts"

Swap to a relative import — the module is now genuinely isomorphic.

js/src/lib/errors.ts

@@ -1,5 +1,5 @@
 import { z } from "zod";
-import { appEnv } from "@/config/env";
+import { appEnv } from "../config/env";
 
 export class ApiError extends Error {
   constructor(