fix(#280): increase cookie expiration from 60s to 24h

[cassio-rossi]

Summary

• Ad banners were reappearing despite valid subscriptions because the patr cookie expired after just 60 seconds
• In v4 (UIKit), this worked because HTTPCookieStorage.shared kept cookies alive process-wide. In v5 (SwiftUI native WebView), cookies are scoped to WKWebsiteDataStore and truly expire after the set interval
• Deferred/lazy-loaded ads that fire after 60s no longer saw the patr cookie, causing banners to reappear
• Increased expiration to 24 hours (86,400s) — cookies are still re-created fresh on every page load, so there's no stale-data risk
• Extracted the expiration value into a cookieExpiration constant for maintainability

Closes #280

Test plan

• Open an article as a subscriber — verify no ads appear
• Stay on the article for >60 seconds — verify ads don't reappear
• Scroll through the article (trigger lazy-loaded content) — verify no ads
• Open multiple articles in sequence — verify no ads on any
• Test without subscription — verify ads DO appear (cookie value is "false")
• Run MacMagazineUILibraryTests — all cookie tests pass ✅

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