fix(#281): preserve article reading position when returning from links

[cassio-rossi]

Summary

Fixes #281 — tapping any link inside an article (external Safari or the inline pushed browser) reloaded the article and lost the reading position on return.

Root causes

1. Inline browser (push/pop): ManagedWebView's .task(id: reloadTrigger) restarts when the covered article reappears and unconditionally re-ran performLoad — a full URL load for articles, since they pass no cacheKey.
2. External Safari: while backgrounding, iOS renders app-switcher snapshots in both appearances, firing colorScheme onChange twice — each flip triggered page?.reload() in MMWebView plus a cookie-update + full reload in ManagedWebView.

Changes

• WebViewLoadPolicy (new): pure decision logic — skip a load whose trigger already completed; ignore color-scheme changes while the scene is not active; reconcile only when the appearance genuinely changed while away.
• ManagedWebView: tracks completedLoadTrigger (skips the redundant pop-back reload; cleared on .webContentProcessTerminated so a jettisoned page recovers) and appliedColorScheme (snapshot flips ignored; a real appearance change is applied once on return to .active).
• MMWebView: same scenePhase gating + reconciliation around page?.reload().
• The isActive scenePhase crash-prevention gating and the removeAds reload path are untouched.

Quality gates

• ✅ Build succeeded (iOS Simulator)
• ✅ xcodebuild test -testPlan MacMagazine passed, including the new WebViewLoadPolicy Tests Swift Testing suite (9 cases)
• ✅ swiftlint lint --strict — 0 violations in 241 files
• ✅ Manually verified on simulator: reading position preserved after returning from external Safari and from the inline browser; foreground light/dark switching still reloads with the correct scheme

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