build: type-check the application program, not the test tree, in next build

next 16.3 type-checks the whole tsconfig program during the build. That
reaches the unit tests under src/**/__tests__, their fixtures under
tests/, the Playwright config and the e2e tree, none of which is in the
image build context — .dockerignore leaves tests/ and e2e/ out on
purpose — so the image build stopped at "Cannot find module
'../../tests/fixtures/...'" on both architectures.

The build now reads tsconfig.build.json, which extends the main tsconfig
and excludes the test tree and the test-runner configs. `pnpm typecheck`
keeps using tsconfig.json and still covers the full program; the change
narrows only what `next build` compiles, which was already the case on
16.2 by omission. Verified by building with tests/ and e2e/ moved away.

next.config.ts

@@ -5,6 +5,12 @@ import { version as PKG_VERSION } from "./package.json";
 
 const nextConfig: NextConfig = {
   output: "standalone",
+  // `next build` type-checks the whole tsconfig program since 16.3, which
+  // reaches the unit tests under `src/**/__tests__` and their fixtures under
+  // `tests/`. The image build context leaves `tests/` and `e2e/` out on
+  // purpose, so the build reads a tsconfig without them; `pnpm typecheck`
+  // still covers the full program.
+  typescript: { tsconfigPath: "tsconfig.build.json" },
   poweredByHeader: false,
   // v1.4.38.4 — expose the package.json version to the client bundle
   // so the `<VersionPoller>` can compare the shell-baked version

tsconfig.build.json

@@ -0,0 +1,17 @@
+{
+  "extends": "./tsconfig.json",
+  "exclude": [
+    "node_modules",
+    "scripts",
+    "tests",
+    "e2e",
+    "src/**/__tests__/**",
+    "src/**/*.test.ts",
+    "src/**/*.test.tsx",
+    "playwright.config.ts",
+    "vitest.config.ts",
+    "vitest.config.mts",
+    "vitest.integration.config.ts",
+    "vitest.integration.config.mts"
+  ]
+}