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.

2 files changed, 2 insertions, 2 deletions

diff --git a/Hutch/Views/Repositories/RepositoryDetailViewModel.swift b/Hutch/Views/Repositories/RepositoryDetailViewModel.swift
index 82e1592..62a8b12 100644
--- a/Hutch/Views/Repositories/RepositoryDetailViewModel.swift
+++ b/Hutch/Views/Repositories/RepositoryDetailViewModel.swift
@@ -557,7 +557,7 @@ final class RepositoryDetailViewModel {
                 variables: [
                     "repoId": repository.id,
                     "revspec": revspec,
-                    "file": nil as String? as Any
+                    "file": nil as String? as any Sendable
                 ],
                 file: MultipartUploadFile(
                     variablePath: "file",
diff --git a/Hutch/Views/Repositories/SyntaxHighlighter.swift b/Hutch/Views/Repositories/SyntaxHighlighter.swift
index f8b2cbb..dfab443 100644
--- a/Hutch/Views/Repositories/SyntaxHighlighter.swift
+++ b/Hutch/Views/Repositories/SyntaxHighlighter.swift
@@ -208,7 +208,7 @@ nonisolated final class SyntaxHighlighter {
     ]
 }
 
-private extension UIColor {
+nonisolated private extension UIColor {
     /// `#rrggbb` for HTML inline styles, or `nil` if the color isn't RGB-convertible.
     var hexRGBString: String? {
         var red: CGFloat = 0