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, 3 deletions

diff --git a/Hutch/Models/Paste.swift b/Hutch/Models/Paste.swift
index b4c8f18..a8bf926 100644
--- a/Hutch/Models/Paste.swift
+++ b/Hutch/Models/Paste.swift
@@ -1,6 +1,6 @@
 import Foundation
 
-struct PasteFile: Codable, Sendable, Hashable, Identifiable {
+nonisolated struct PasteFile: Codable, Sendable, Hashable, Identifiable {
     let filename: String?
     let hash: String
     let contents: URL?
@@ -8,7 +8,7 @@ struct PasteFile: Codable, Sendable, Hashable, Identifiable {
     var id: String { hash }
 }
 
-struct Paste: Codable, Sendable, Identifiable, Hashable {
+nonisolated struct Paste: Codable, Sendable, Identifiable, Hashable {
     let id: String
     let created: Date
     let visibility: Visibility
@@ -16,7 +16,7 @@ struct Paste: Codable, Sendable, Identifiable, Hashable {
     let user: Entity
 }
 
-struct PasteUploadDraft: Identifiable, Equatable, Sendable {
+nonisolated struct PasteUploadDraft: Identifiable, Equatable, Sendable {
     let id: UUID
     var filename: String
     var contents: String