Adopt Swift 6 language mode

Flip SWIFT_VERSION to 6.0 (keeping SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor)
and resolve every resulting diagnostic. Mechanical, no behaviour change:

- Models: pure value types in Models/ and utility extensions (Date, DateFormatter,
  SRHTWebURL) marked nonisolated so the nonisolated networking layer can use them.
- App Intents: stored 'static var' (title/description/openAppWhenRun/
  typeDisplayRepresentation/defaultQuery/caseDisplayRepresentations) -> 'static let';
  drop the invalid @retroactive on the in-module LookupType: AppEnum.
- Sendable dicts: 'nil as String? as Any' -> 'nil as String? as any Sendable'.
- UserDefaults: retroactive @unchecked Sendable (documented thread-safe).
- WidgetKit completion handlers rebound nonisolated(unsafe) to cross into Task {};
  TipStoreViewModel task handle is @ObservationIgnored nonisolated(unsafe) for
  its nonisolated deinit.
- Tests run on @MainActor; a few constant fixtures marked nonisolated for use in
  @Sendable stub closures.

Builds and the full suite are clean in Swift 6 mode. Roadmap updated.

1 files changed, 3 insertions, 1 deletions

diff --git a/Hutch/Views/More/TipStoreViewModel.swift b/Hutch/Views/More/TipStoreViewModel.swift
index bc909c1..043b218 100644
--- a/Hutch/Views/More/TipStoreViewModel.swift
+++ b/Hutch/Views/More/TipStoreViewModel.swift
@@ -32,7 +32,9 @@ final class TipStoreViewModel {
     var errorMessage: String?
     var statusMessage: String?
 
-    private var transactionUpdatesTask: Task<Void, Never>?
+    // Internal task handle, not observable state; assigned only on the main
+    // actor and read once from the nonisolated deinit.
+    @ObservationIgnored nonisolated(unsafe) private var transactionUpdatesTask: Task<Void, Never>?
 
     init() {
         transactionUpdatesTask = Task.detached(priority: .background) {