Add offline resilience to subscription status checks

Cache subscription expiration date so premium access survives
offline launches and transient StoreKit failures. Restores cached
state synchronously on init to eliminate loading flash.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Shared/Models/UserDefaultsStore.swift

@@ -196,6 +196,7 @@ class UserDefaultsStore {
         case daysFilter
         case firstLaunchDate
         case hasActiveSubscription
+        case cachedSubscriptionExpiration
         case lastVotedDate
         case votingLayoutStyle
         case dayViewStyle