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/DayView/DayView.swift

@@ -159,6 +159,7 @@ extension DayView {
                 defaultSectionHeader(month: month, year: year)
             }
         }
+        .accessibilityIdentifier(AccessibilityID.DaySection.header(month: month, year: year))
     }
 
     private func defaultSectionHeader(month: Int, year: Int) -> some View {