Add XCUITest suite with 27 test files covering unmapped P1 test cases

- Add 8 new test files: HeaderMoodLogging (TC-002), DayViewGrouping (TC-019),
  AllDayViewStyles (TC-021), MonthViewInteraction (TC-030), PaywallGate
  (TC-032/039/048), AppTheme (TC-070), IconPack (TC-072),
  PremiumCustomization (TC-075)
- Add accessibility IDs for paywall overlays, icon packs, app theme cards,
  and day view section headers
- Add --expire-trial launch argument to UITestMode for paywall gate testing
- Update QA test plan spreadsheet with XCUITest names for 14 test cases
- Include existing test infrastructure: screen objects, helpers, base class,
  and 19 previously written test files

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

Shared/Views/MainTabView.swift

@@ -28,26 +28,31 @@ struct MainTabView: View {
                 .tabItem {
                     Label(String(localized: "content_view_tab_main"), systemImage: "list.dash")
                 }
+                .accessibilityIdentifier(AccessibilityID.Tab.day)
 
             monthView
                 .tabItem {
                     Label(String(localized: "content_view_tab_month"), systemImage: "calendar")
                 }
+                .accessibilityIdentifier(AccessibilityID.Tab.month)
 
            yearView
                 .tabItem {
                     Label(String(localized: "content_view_tab_filter"), systemImage: "line.3.horizontal.decrease.circle")
                 }
+                .accessibilityIdentifier(AccessibilityID.Tab.year)
 
             insightsView
                 .tabItem {
                     Label(String(localized: "content_view_tab_insights"), systemImage: "lightbulb.fill")
                 }
+                .accessibilityIdentifier(AccessibilityID.Tab.insights)
 
             SettingsTabView()
                 .tabItem {
                     Label("Settings", systemImage: "gear")
                 }
+                .accessibilityIdentifier(AccessibilityID.Tab.settings)
         }
         .accentColor(textColor)
         .sheet(isPresented: $needsOnboarding, onDismiss: { }, content: {