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.

Model/NavigationModel.swift

@@ -318,7 +318,7 @@ final class NavigationModel: ObservableObject {
     func multipleTapHandler() {
         switch tabSelection {
         case .search:
-            search.focused = true
+            break
         default:
             print("not implemented")
         }