Tests pass under heavy host load (closes #233)

The suite timed out when this host was heavily loaded. vitest now runs at most four workers instead of one per CPU core. `test/cli/bin.test.ts` loads the sources `bin/quak.ts` imports and starts libsodium once, before its tests, so no test pays for the first load. The fake accounts in `test/ente-server.ts` and `test/thumbnails/thumbnails.test.ts` stretch their passwords at libsodium's lowest Argon2id cost, since every login stretches them again.

Judgement call: the login, usage, unwrap and KDF tests keep the realistic 64 MiB cost.

Model: opus-5-5

vitest.config.ts

@@ -3,8 +3,15 @@ import { configDefaults, defineConfig } from "vitest/config";
 // vitest does not read .gitignore when looking for tests. A checkout nested
 // under .claude/ has its own test/ tree, and without this exclude the suite
 // runs once per nested checkout and still reports success.
+//
+// vitest starts one worker per CPU core by default, and a shared host has
+// dozens. Four keep the suite well inside the Dockerfile's time limit. vitest
+// 2 rejects a maxWorkers below its default minWorkers, so minWorkers is set
+// too.
 export default defineConfig({
     test: {
         exclude: [...configDefaults.exclude, ".claude/**"],
+        minWorkers: 1,
+        maxWorkers: 4,
     },
 });