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, 2 insertions, 2 deletions

diff --git a/Hutch/Models/RepositoryACL.swift b/Hutch/Models/RepositoryACL.swift
index e5f5eb1..e5d93f6 100644
--- a/Hutch/Models/RepositoryACL.swift
+++ b/Hutch/Models/RepositoryACL.swift
@@ -1,12 +1,12 @@
 import Foundation
 
-struct RepositoryACLEntry: Codable, Sendable, Identifiable, Hashable {
+nonisolated struct RepositoryACLEntry: Codable, Sendable, Identifiable, Hashable {
     let id: Int
     let mode: AccessMode
     let entity: Entity
 }
 
-extension AccessMode {
+nonisolated extension AccessMode {
     var shortLabel: String { rawValue }
 
     var displayName: String {