keymap_editor: Fix filter input element alignment (#38895)

# Why

I have spotted that Keymap Editor filter input (editor) is misaligned
vertically.

# How

Switch the input wrapper to flex layout, use `items_center` to align
editor vertically in center of the wrapper.

Release Notes:

- Fixed Keymap Editor filter input alignment

# Test plan

I have tested the change locally and compared the UI before and after,
to make sure that change does not affect the size of the wrapper
element.

### Before

<img width="1622" height="428" alt="Screenshot 2025-09-25 at 18 18 59"
src="https://github.com/user-attachments/assets/7d09be5c-6caf-4873-8ecf-2542851cb40a"
/>

### After

<img width="1622" height="428" alt="Screenshot 2025-09-25 at 18 07 18"
src="https://github.com/user-attachments/assets/540fcb3e-691d-4fb7-8130-2ed45ddc0adc"
/>

crates/keymap_editor/src/keymap_editor.rs

@@ -1569,7 +1569,7 @@ impl Render for KeymapEditor {
                         h_flex()
                             .gap_2()
                             .child(
-                                div()
+                                h_flex()
                                     .key_context({
                                         let mut context = KeyContext::new_with_defaults();
                                         context.add("BufferSearchBar");