v4.7.0: Add domain-vs-domain comparison

- DiffService.compare(domainA:domainB:) reuses the existing section-diff
  builders to compare two distinct domains' latest reports, returning a new
  DomainComparisonResult (parallel to the time-based DomainDiff).
- DomainCompareView: pick two tracked domains and render the comparison with
  the existing DomainDiffView section renderer.
- Entry point: "Compare Domains" in the Watchlist toolbar menu, shown once at
  least two domains are tracked.

1 files changed, 82 insertions, 0 deletions

diff --git a/DomainDig/DomainCompareView.swift b/DomainDig/DomainCompareView.swift
new file mode 100644
index 0000000..96341c2
--- /dev/null
+++ b/DomainDig/DomainCompareView.swift
@@ -0,0 +1,82 @@
+import SwiftUI
+
+/// Side-by-side comparison of two tracked domains' latest reports, reusing the
+/// same section-diff rendering (`DomainDiffView`) that time-based history diffs
+/// use, via `DiffService.compare(domainA:domainB:)`.
+struct DomainCompareView: View {
+    @Bindable var viewModel: DomainViewModel
+    @State private var domainAID: UUID?
+    @State private var domainBID: UUID?
+
+    private var states: [PortfolioDomainStatus] {
+        viewModel.portfolioDashboardData.domainStates
+    }
+
+    private var stateA: PortfolioDomainStatus? {
+        states.first { $0.trackedDomain.id == domainAID }
+    }
+
+    private var stateB: PortfolioDomainStatus? {
+        states.first { $0.trackedDomain.id == domainBID }
+    }
+
+    private var result: DomainComparisonResult? {
+        guard let stateA, let stateB, stateA.trackedDomain.id != stateB.trackedDomain.id else {
+            return nil
+        }
+        return DiffService.compare(domainA: stateA.report, domainB: stateB.report)
+    }
+
+    var body: some View {
+        List {
+            Section("Domains") {
+                domainPicker("Domain A", selection: $domainAID)
+                domainPicker("Domain B", selection: $domainBID)
+            }
+
+            if domainAID != nil, domainAID == domainBID {
+                Section {
+                    Text("Choose two different domains to compare.")
+                        .font(.callout)
+                        .foregroundStyle(.secondary)
+                }
+            }
+
+            if let result {
+                Section {
+                    DomainDiffView(
+                        title: "\(result.domainA) vs \(result.domainB)",
+                        sections: result.sections,
+                        contextNote: result.contextNote,
+                        showsUnchanged: true,
+                        highlightedSectionID: nil
+                    )
+                }
+                .listRowBackground(Color.clear)
+            } else if domainAID == nil || domainBID == nil {
+                Section {
+                    EmptyStateCardView(
+                        title: "Pick Two Domains",
+                        message: "Select a domain in each slot to see a side-by-side comparison across DNS, TLS, ownership, and risk.",
+                        suggestion: "Both domains must already be tracked.",
+                        systemImage: "arrow.left.arrow.right",
+                        showsCardBackground: false
+                    )
+                }
+                .listRowBackground(Color(.systemGray6).opacity(0.5))
+            }
+        }
+        .scrollContentBackground(.hidden)
+        .background(Color.black)
+        .navigationTitle("Compare Domains")
+    }
+
+    private func domainPicker(_ label: String, selection: Binding<UUID?>) -> some View {
+        Picker(label, selection: selection) {
+            Text("Select a domain").tag(UUID?.none)
+            ForEach(states) { state in
+                Text(state.trackedDomain.domain).tag(Optional(state.trackedDomain.id))
+            }
+        }
+    }
+}