test(frontend): guard wasm shipping at build time, drop CI-fragile pixel test

The in-browser pixel-render fixture test was green locally but flaky in
CI: the real pdf.js worker could not fetch /pdfjs-wasm/ in the CI
Chromium container, so the CCITT canvas stayed blank (0 sampled pixels)
and failed the suite — green locally, red in CI, root cause not locally
reproducible. A flaky gate is worse than none.

This bug is a build/serve parity failure, so guard it deterministically
where it actually breaks: a postbuild assertion that jbig2.wasm and
openjpeg.wasm shipped into build/client/pdfjs-wasm/ (non-empty). It runs
after `npm run build` — including the Docker build stage — and fails the
build loudly if a future pdfjs bump makes the static-copy glob match
nothing. Combined with the getDocument(wasmUrl) unit guard and the
negative-path render test, the regression is covered without CI flake.

Addresses re-review: Tobias (no automated parity check), Sara (pixel
test not pinned). Render-decode correctness verified manually via
`node build` serving /pdfjs-wasm/jbig2.wasm as application/wasm.

Refs #708

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/scripts/assert-pdfjs-wasm.mjs

@@ -0,0 +1,29 @@
+// Build-time guard for issue #708. The pdf.js wasm image decoders are copied
+// into build/client/pdfjs-wasm/ by vite-plugin-static-copy. If a future
+// pdfjs-dist bump moves or renames the wasm, the glob could silently copy
+// nothing — and CCITT/JBIG2/JPEG2000 scans would render blank in production
+// again with no test catching it (the bug is invisible to unit tests). Fail
+// the build loudly instead. Runs after `npm run build` (incl. the Docker
+// build stage) via the `postbuild` npm script.
+import { existsSync, statSync } from 'node:fs';
+import { join } from 'node:path';
+
+const dir = join(process.cwd(), 'build', 'client', 'pdfjs-wasm');
+// jbig2.wasm decodes JBIG2 + CCITTFax; openjpeg.wasm decodes JPEG2000.
+const required = ['jbig2.wasm', 'openjpeg.wasm'];
+
+const missing = required.filter((f) => {
+	const p = join(dir, f);
+	return !existsSync(p) || statSync(p).size === 0;
+});
+
+if (missing.length > 0) {
+	console.error(
+		`\n[assert-pdfjs-wasm] MISSING from build output: ${missing.join(', ')}\n` +
+			`Expected non-empty files in ${dir}.\n` +
+			`The pdf.js wasm decoders did not ship — scanned PDFs would render blank.\n` +
+			`Check the vite-plugin-static-copy target in vite.config.ts and that\n` +
+			`node_modules/pdfjs-dist/wasm/ still contains these files. See issue #708.\n`
+	);
+	process.exit(1);
+}