Scope dark mode change only to the presenting sheet (#6144)

* Scope dark mode change only to the presenting sheet, instead of the whole window

* Apply review suggestions to the presentation colour scheme modifier

API:
- Take a `ColorScheme?` at the SwiftUI level instead of a `UIUserInterfaceStyle`
  and map it once inside the representable (`nil` means no override).
- Rename to `presentationColorScheme` and the file to match.
- Restore the `preferredColorScheme` property on the media preview details and
  redact confirmation views, and the `nil` at the media timeline call site, so
  no UIKit types leak into the views.

Cleanup:
- Walk the responder chain with `sequence(first:next:)` instead of a while loop.

* Re-record the media upload preview iPad snapshots

The Cancel button no longer has an explicit tint, so its label colour changed
very slightly from `textActionPrimary` to the default. Only the iPad references
include the toolbar, which is why the iPhone ones were unaffected.

ElementX/Sources/Screens/FilePreviewScreen/View/TimelineMediaPreviewRedactConfirmationView.swift

@@ -38,7 +38,7 @@ struct TimelineMediaPreviewRedactConfirmationView: View {
         .presentationDetents([.height(sheetHeight + topPadding)])
         .presentationDragIndicator(.visible)
         .presentationBackground(.compound.bgCanvasDefault)
-        .preferredColorScheme(preferredColorScheme)
+        .presentationColorScheme(preferredColorScheme)
     }
     
     private var header: some View {