fix(platform-web): expire a cookie immediately with maxAge: 0

[dangreen]

Why

CookieStorage.set turned maxAge into expires behind a truthiness check:

expires: maxAge ? Date.now() + maxAge * 1000 : expires

maxAge: 0 means immediate expiration in Set-Cookie terms, but here it read as "not set", so the cookie was written with the expires option instead, usually as a session cookie. The new spec should expire the cookie immediately with zero max age times out on main waiting for the cookie to disappear.

What

• The check is now maxAge === undefined, so zero expires the cookie right away. maxAge keeps its precedence over expires, as in RFC 6265.
• Two browser tests against the real Cookie Store: maxAge: 60 lands expires within a minute from now, maxAge: 0 deletes the cookie.
• One sentence on the Cookies docs page: maxAge is given in seconds and takes precedence over expires.

Size

All publics grows by 3 B gzipped for the undefined comparison, 3419 B against the 3.45 kB limit re-pinned in #247.

Checks

• oxlint, tsc --noEmit, vitest run (62 tests, browser mode) and size-limit in packages/platform-web pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.33%. Comparing base (2696386) to head (9b83d02).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #248      +/-   ##
==========================================
+ Coverage   83.29%   83.33%   +0.03%     
==========================================
  Files          98       98              
  Lines        2556     2556              
  Branches      551      551              
==========================================
+ Hits         2129     2130       +1     
  Misses        314      314              
+ Partials      113      112       -1     

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