chore(deps): remove the fast-text-encoding polyfill

[vbudhram]

Because

• TextEncoder and TextDecoder are globals in every Node and browser target FxA supports, so the fast-text-encoding polyfill never loads.
• The import only ran for its side effect. Nothing in the repo imports a name from it.

This pull request

• Removes the typeof TextEncoder === 'undefined' guard and its dynamic import from AuthClient.create in client.ts.
• Removes the fast-text-encoding resolve alias from webpack.config.js.
• Drops fast-text-encoding and @types/fast-text-encoding from the fxa-auth-client and fxa-content-server manifests, and updates yarn.lock to match.

Issue that this pull request solves

Closes: https://mozilla-hub.atlassian.net/browse/FXA-13921

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

How to review (Optional)

• Key files/areas to focus on: AuthClient.create in packages/fxa-auth-client/lib/client.ts.
• Suggested review order: client.ts, then the webpack alias, then the two manifests and the lockfile.
• Risky or complex parts: the published-package note below. The premise that TextEncoder exists in all supported targets is the reviewer's to confirm.

Screenshots (Optional)

Other information (Optional)

packages/fxa-auth-client publishes to npm, so third parties consume it. Today, on an environment with no global TextEncoder, AuthClient.create() recovers by loading the polyfill. After this change it does not, and such a consumer fails later inside checkWebCrypto or during key derivation. Nothing replaces the guard, since the ticket asks for removal rather than a substitute. No exported signature moves, so this is not an API change.

Verified locally: nx lint fxa-auth-client and nx lint fxa-content-server pass, tsc -p packages/fxa-auth-client/tsconfig.json --noEmit is clean, and the fxa-auth-client unit suite passes (47 tests). No tests were added or changed. Functional tests were not run.

[Copilot]

Pull request overview

Removes an obsolete text-encoding polyfill from the FxA auth client and legacy content server.

Changes:

• Removes conditional polyfill loading from AuthClient.create.
• Removes the webpack alias and package dependencies.
• Updates the lockfile accordingly.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/fxa-auth-client/lib/client.ts	Removes dynamic polyfill loading.
packages/fxa-auth-client/package.json	Drops polyfill packages.
packages/fxa-content-server/package.json	Drops the runtime dependency.
packages/fxa-content-server/webpack.config.js	Removes the obsolete alias.
yarn.lock	Removes dependency resolutions.

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