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.

5 files changed, 11 insertions, 9 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
diff --git a/Hutch/Views/More/TipStoreViewModel.swift b/Hutch/Views/More/TipStoreViewModel.swift
index bc909c1..043b218 100644
--- a/Hutch/Views/More/TipStoreViewModel.swift
+++ b/Hutch/Views/More/TipStoreViewModel.swift
@@ -32,7 +32,9 @@ final class TipStoreViewModel {
     var errorMessage: String?
     var statusMessage: String?
 
-    private var transactionUpdatesTask: Task<Void, Never>?
+    // Internal task handle, not observable state; assigned only on the main
+    // actor and read once from the nonisolated deinit.
+    @ObservationIgnored nonisolated(unsafe) private var transactionUpdatesTask: Task<Void, Never>?
 
     init() {
         transactionUpdatesTask = Task.detached(priority: .background) {
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
diff --git a/Hutch/Views/Settings/SettingsViewModel.swift b/Hutch/Views/Settings/SettingsViewModel.swift
index 8536e60..2d57ba5 100644
--- a/Hutch/Views/Settings/SettingsViewModel.swift
+++ b/Hutch/Views/Settings/SettingsViewModel.swift
@@ -198,9 +198,9 @@ final class SettingsViewModel {
         do {
             let input: [String: any Sendable] = [
                 "email": email,
-                "url": url.isEmpty ? nil as String? as Any : url,
-                "location": location.isEmpty ? nil as String? as Any : location,
-                "bio": bio.isEmpty ? nil as String? as Any : bio
+                "url": url.isEmpty ? nil as String? as any Sendable : url,
+                "location": location.isEmpty ? nil as String? as any Sendable : location,
+                "bio": bio.isEmpty ? nil as String? as any Sendable : bio
             ]
             let result = try await client.execute(
                 service: .meta,
@@ -243,7 +243,7 @@ final class SettingsViewModel {
         do {
             // The input variable has avatar set to null; the actual file
             // is sent as a separate multipart part per graphql-multipart-request-spec.
-            let input: [String: any Sendable] = ["avatar": nil as String? as Any]
+            let input: [String: any Sendable] = ["avatar": nil as String? as any Sendable]
             let result = try await client.executeMultipart(
                 service: .meta,
                 query: Self.updateUserMutation,
@@ -286,7 +286,7 @@ final class SettingsViewModel {
         error = nil
 
         do {
-            let input: [String: any Sendable] = ["avatar": nil as String? as Any]
+            let input: [String: any Sendable] = ["avatar": nil as String? as any Sendable]
             let result = try await client.execute(
                 service: .meta,
                 query: Self.updateUserMutation,