Fix iPad iOS 18 keyboard dismissal issue in search

Removed auto-focus logic that was causing keyboard show/hide loop
on iPad with docked keyboard. The keyboard was repeatedly dismissing
immediately after appearing due to interaction between keyboard
notifications, focus state changes, and view updates.

Changes:
- Removed focused state and keyboard observer from SearchModel
- Removed iOS textField reference (kept macOS only)
- Removed auto-focus logic from FocusableSearchTextField on iOS
- Cleaned up unused focus-related code

The search field now works reliably when tapped manually on iPad.
Auto-focus still works on macOS where it doesn't cause issues.

Shared/Search/SearchView.swift

@@ -49,10 +49,10 @@ struct SearchView: View {
                                 .opacity(state.queryText.isEmpty ? 0 : 1)
                         } else {
                             results
+                                .backport
+                                .scrollDismissesKeyboardInteractively()
                         }
                     }
-                    .backport
-                    .scrollDismissesKeyboardInteractively()
                 }
                 .environment(\.listingStyle, searchListingStyle)
                 .toolbar {