fix(build): make the production build pass, and stop it eating 20GB

`next build` had never completed on this host, so three real defects were
sitting in the tree untested. All three are now fixed and the build is green.

- The build was not memory-bound the way it looked. Turbopack's builder reached
  20.5GB RSS and died, and raising `--max-old-space-size` could never have
  helped: that flag caps the V8 heap, while the 20GB sat in Turbopack's own Rust
  allocator. The first symptom was misleading because the process doing the
  allocating is a grandchild of `npx`, so watching the direct child shows a
  95MB shim the whole time. Building with `--webpack` puts the build back under
  the JS heap, where the flag actually applies: peak 5.9GB, 150s, exit 0.

- withAdmin's second parameter was typed `{ params?: ... }` and given a `= {}`
  default, which made it optional and `RouteContext | undefined`. Next's
  generated route types assert that argument against `ParamCheck<RouteContext>`
  and reject it, across 113 route files. `tsc --noEmit` cannot see this, because
  Next only adds `.next/types` to the project during a production build — so the
  type check that everyone runs locally was structurally incapable of catching
  the only type error that blocks a deploy. `params` is now required, which is
  also what the code already assumed: it is awaited with no guard. The 35 test
  call sites that invoked a handler with one argument now pass a real context,
  and the await got a guard so a direct internal call cannot turn a missing
  context into a 500.

- `src/app/api/admin/import/furni/route.ts` re-exported `ensureDirectories` and
  `importSingleFurni` for "backward compatibility" that nothing used; the batch
  route imports from `@/lib/services/furni-import` directly. Next rejects any
  value export from a route module that is not an HTTP verb or config, so this
  had been breaking the build for as long as it existed. Removed.

- `isomorphic-dompurify` builds its server-side DOM through jsdom. Bundled, that
  pulls jsdom's `browser/default-stylesheet.css` into the server chunk, where the
  path no longer resolves, and page-data collection dies with ENOENT on every
  page that sanitizes HTML. Marked external so Node resolves it from
  node_modules and the standalone tracer includes it.

The remaining build warning is a pre-existing circular dependency between
chunks that share the webpack runtime. It costs hash reuse, not correctness, and
is left alone rather than churned here.

Verified: build exit 0, 276 static pages generated, 3223 tests pass, tsc and
biome clean.

next.config.ts

@@ -36,7 +36,18 @@ const nextConfig: NextConfig = {
 	reactStrictMode: true,
 	compress: true,
 	productionBrowserSourceMaps: false,
-	serverExternalPackages: ["lzma-wasm", "sharp", "pino", "pino-pretty"],
+	// `isomorphic-dompurify` builds a DOM through jsdom on the server. Bundled,
+	// it drags jsdom's `browser/default-stylesheet.css` into the server chunk,
+	// where the path no longer exists and page-data collection dies with ENOENT
+	// on any page that sanitizes HTML. Kept external, Node resolves it from
+	// node_modules at runtime and the standalone output traces it in.
+	serverExternalPackages: [
+		"lzma-wasm",
+		"sharp",
+		"pino",
+		"pino-pretty",
+		"isomorphic-dompurify",
+	],
 
 	async redirects() {
 		return [