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.

4 files changed, 10 insertions, 3 deletions

diff --git a/Hutch/Extensions/Date+Relative.swift b/Hutch/Extensions/Date+Relative.swift
index f06ba59..b6c28ed 100644
--- a/Hutch/Extensions/Date+Relative.swift
+++ b/Hutch/Extensions/Date+Relative.swift
@@ -1,6 +1,6 @@
 import Foundation
 
-extension Date {
+nonisolated extension Date {
     /// A short relative description like "2h ago", "3d ago", or "Jan 5, 2025".
     var relativeDescription: String {
         let formatter = RelativeDateTimeFormatter()
diff --git a/Hutch/Extensions/DateFormatter+SRHT.swift b/Hutch/Extensions/DateFormatter+SRHT.swift
index c017b11..a06b58a 100644
--- a/Hutch/Extensions/DateFormatter+SRHT.swift
+++ b/Hutch/Extensions/DateFormatter+SRHT.swift
@@ -1,6 +1,6 @@
 import Foundation
 
-extension DateFormatter {
+nonisolated extension DateFormatter {
     /// Formatter for the sr.ht `Time` scalar: `%Y-%m-%dT%H:%M:%SZ` (UTC).
     static let srht: DateFormatter = {
         let formatter = DateFormatter()
diff --git a/Hutch/Extensions/Foundation+Sendable.swift b/Hutch/Extensions/Foundation+Sendable.swift
new file mode 100644
index 0000000..9a12aa6
--- /dev/null
+++ b/Hutch/Extensions/Foundation+Sendable.swift
@@ -0,0 +1,7 @@
+import Foundation
+
+// `UserDefaults` is documented as thread-safe but is not marked `Sendable` by
+// Foundation. Hutch passes it into account sessions and the various per-account
+// stores, all of which need to cross concurrency boundaries. The retroactive
+// unchecked conformance reflects the documented thread-safety.
+extension UserDefaults: @retroactive @unchecked Sendable {}
diff --git a/Hutch/Extensions/SRHTWebURL.swift b/Hutch/Extensions/SRHTWebURL.swift
index fc5a1d5..5715ce4 100644
--- a/Hutch/Extensions/SRHTWebURL.swift
+++ b/Hutch/Extensions/SRHTWebURL.swift
@@ -1,6 +1,6 @@
 import Foundation
 
-enum SRHTWebURL {
+nonisolated enum SRHTWebURL {
     static let chat = URL(string: "https://chat.sr.ht")!
     static let status = URL(string: "https://status.sr.ht")!
     /// RSS/Atom feed for status incidents (HTML snapshot uses `status`).