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/SearchTextField.swift

@@ -55,9 +55,9 @@ struct SearchTextField: View {
                 TextField("Search...", text: $state.queryText) {
                     state.changeQuery { query in
                         query.query = state.queryText
-                        navigation.hideKeyboard()
                     }
                     RecentsModel.shared.addQuery(state.queryText)
+                    navigation.hideKeyboard()
                 }
                 .disableAutocorrection(true)
                 .textFieldStyle(.plain)