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, 1 insertions, 1 deletions

diff --git a/Hutch/Views/Lists/MailingListListView.swift b/Hutch/Views/Lists/MailingListListView.swift
index 1b159bf..b6ed2ff 100644
--- a/Hutch/Views/Lists/MailingListListView.swift
+++ b/Hutch/Views/Lists/MailingListListView.swift
@@ -100,7 +100,7 @@ final class MailingListListViewModel {
                 query: Self.createMailingListMutation,
                 variables: [
                     "name": trimmedName,
-                    "description": trimmedDescription.isEmpty ? nil as String? as Any : trimmedDescription,
+                    "description": trimmedDescription.isEmpty ? nil as String? as any Sendable : trimmedDescription,
                     "visibility": visibility.rawValue
                 ],
                 responseType: Response.self