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, 4 insertions, 0 deletions

diff --git a/HutchWidgetExtension/ContributionGraphWidget.swift b/HutchWidgetExtension/ContributionGraphWidget.swift
index 2fc07f2..bc56a03 100644
--- a/HutchWidgetExtension/ContributionGraphWidget.swift
+++ b/HutchWidgetExtension/ContributionGraphWidget.swift
@@ -28,12 +28,16 @@ struct ContributionGraphTimelineProvider: TimelineProvider {
     }
 
     func getSnapshot(in _: Context, completion: @escaping (ContributionGraphEntry) -> Void) {
+        // WidgetKit's completion is safe to call from the task; rebind as
+        // nonisolated(unsafe) so it can cross into the `sending` Task closure.
+        nonisolated(unsafe) let completion = completion
         Task {
             completion(await loadEntry())
         }
     }
 
     func getTimeline(in _: Context, completion: @escaping (Timeline<ContributionGraphEntry>) -> Void) {
+        nonisolated(unsafe) let completion = completion
         Task {
             let entry = await loadEntry()
             let refreshDate = Calendar.contributionCalendar.date(byAdding: .hour, value: 1, to: Date()) ?? Date().addingTimeInterval(3600)