feat(v2.0.0): introduce tracked domains, diffing, and monitoring foundationsv2.0.0

- replace basic watchlist with structured TrackedDomain model
- add manual refresh flow for tracked domains
- implement snapshot diffing (DNS, IP, TLS, HTTP, redirect, email, availability)
- add change summaries (changed/unchanged + affected sections)
- link tracked domains to history snapshots
- enhance HistoryEntry with normalized summary fields
- persist availability + suggestions in history and export
- add WatchlistView with pinning, notes, and status display
- introduce premium capability scaffolding (no StoreKit)
- enforce free-tier tracked domain limit (local only)
- refactor view model to support tracking, diffing, and refresh flows

notes:
- no background monitoring, notifications, or paid APIs yet
- all features remain local-first and manual

1 files changed, 27 insertions, 0 deletions

diff --git a/DomainDig/PremiumAccessService.swift b/DomainDig/PremiumAccessService.swift
new file mode 100644
index 0000000..698a2d2
--- /dev/null
+++ b/DomainDig/PremiumAccessService.swift
@@ -0,0 +1,27 @@
+import Foundation
+
+enum PremiumAccessService {
+    static let freeTrackedDomainLimit = 3
+
+    static func hasAccess(to capability: PremiumCapability) -> Bool {
+        switch capability {
+        case .unlimitedTrackedDomains,
+             .automatedMonitoring,
+             .pushAlerts,
+             .batchTracking,
+             .advancedExports:
+            return false
+        }
+    }
+
+    static func trackedDomainLimitMessage(currentCount: Int) -> String? {
+        guard currentCount >= freeTrackedDomainLimit, !hasAccess(to: .unlimitedTrackedDomains) else {
+            return nil
+        }
+        return "More tracked domains will be available in a future Pro upgrade."
+    }
+
+    static func canAddTrackedDomain(currentCount: Int) -> Bool {
+        hasAccess(to: .unlimitedTrackedDomains) || currentCount < freeTrackedDomainLimit
+    }
+}