fix: refresh useCookie value when cookie name changes

[Sudashiii]

Description

Changing the cookieName passed to useCookie leaves the returned value pointing to the previous cookie. For example, switching from useCookie('cookieA') to useCookie('cookieB') continues to return cookie A's value even though the update and delete callbacks already target cookie B.

Refresh the stored value in an effect whenever cookieName changes, returning null when the new cookie does not exist. Add regression tests for switching names, missing cookies, switching back, and update/delete callbacks after switching. Document the behavior.

Fixes #2590.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Validation

• Before the fix, three new regression tests failed with the previous cookie's value.
• After the fix, all 496 tests across 76 suites pass.
• All 8 useCookie tests pass, with 100% statement, branch, function, and line coverage for src/useCookie.ts.
• Type checking and CommonJS/ES module builds pass.
• Lint passes with no errors; 97 existing warnings are in untouched files. The changed source and test files lint cleanly.

Checklist

• Read the Contributing Guide
• Perform a code self-review
• Comment the code, particularly in hard-to-understand areas — not applicable to this small effect
• Add documentation
• Add hook's story at Storybook — not applicable; existing hook API is unchanged
• Cover changes with tests
• Ensure the test suite passes
• Provide 100% test coverage for the changed hook
• Make sure code lints
• Make sure types are fine