fix(platform-web): accept null in parseLocales for a missing Accept-Language header

[dangreen]

Why

Headers.get('accept-language') returns null when the request has no Accept-Language header. parseLocales(acceptLanguage = '') only defaulted undefined and then read acceptLanguage.length, so the example from the docs, parseLocales(request.headers.get('accept-language')), threw TypeError: Cannot read properties of null (reading 'length') on the first request without the header (curl, bots, health checks). Under strict TypeScript the example did not even type-check, since string | null is not assignable to string | undefined.

The built-in SSR renderer was not affected: it passes headers?.acceptLanguage, which is typed string | undefined.

What

• parseLocales now takes acceptLanguage?: string | null and normalizes it with acceptLanguage ||= '', so null behaves like undefined and '': { language: 'en', languages: ['en'] }.
• eslint/no-param-reassign is turned off for the package in oxlint.config.ts, the same way agera does it. A local const would have cost 4 B of gzip and pushed the package over its 3.4 kB size limit; the reassignment keeps it at 3399 B.
• New test: should fallback to english when header is null.

Checks

• oxlint, tsc --noEmit, vitest run (58 tests) in packages/platform-web pass.
• size-limit: 3399 B gzip (limit 3400 B), 3027 B brotli (limit 3.05 kB).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.32%. Comparing base (13a2eb3) to head (3b0973e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
+ Coverage   83.28%   83.32%   +0.03%     
==========================================
  Files          98       98              
  Lines        2554     2554              
  Branches      549      549              
==========================================
+ Hits         2127     2128       +1     
  Misses        314      314              
+ Partials      113      112       -1     

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