fix(test): repair frontend test suite under webpack 5

[jaschadub]

Summary

The frontend puppeteer suite (npm test) has been red on master since the webpack 5 upgrade (#30), which explicitly deferred running it. webpack 5 dropped automatic Node-core polyfills, so the browser test bundle no longer compiled/ran. This was also the blocker that forced #31 and #32 to be pushed with --no-verify.

Changes

Polyfill assert + process (test bundle only)
Every test file import assert from 'assert'. Added assert via resolve.fallback and process via ProvidePlugin (the assert polyfill references process at runtime). Both are lazy/test-only — verified absent from the production app bundle (app.js size unchanged, no process/browser in prod output).

Freeze the http_ece reference as a golden vector
streaming-tests.js used http_ece (a Node-only lib) as a live reference oracle for ECE encryption. Running it in-browser dragged in node:crypto → crypto-browserify → asn1.js → vm, and then failed at runtime because the buffer polyfill can't decode base64url (which http_ece uses for its key/salt). Replaced the live call with the http_ece reference ciphertext frozen as a golden vector (ECE/aes128gcm per RFC 8188 is a stable format). This drops the crypto-browserify / stream-browserify / vm-browserify polyfills entirely — http_ece was their only consumer.

Remove dead reportLink in fileReceiver.js
It imported a symbol api.js never exported (git history shows reportLink was never in api.js). webpack 4 silently made it undefined; webpack 5 errors. Nothing calls the method and there's no server report route — leftover from upstream Firefox Send. Removed the import and the unused method.

Test plan

• npm test — green: 48 backend + 23 frontend passing, 0 failing (this PR was pushed with the pre-push hook, no --no-verify)
• npm run build — both targets green
• npm run lint:js / lint:css — 0 errors
• prod app.js unchanged in size; process/assert polyfills confirmed test-bundle-only
• full upload + password-protected download E2E passes (covers the fileReceiver.js change)

Recommend merging this first; it un-reds npm test so #31 (audit) and #32 (mobile modal) get clean CI.