fix(ui): polish editor layouts and confirm scene delete

Stretch key action buttons, center empty audio/image pickers, and require confirmation before deleting a scene.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/renderer/editor/MaterialsBrowser.tsx

@@ -114,9 +114,11 @@ export function MaterialsBrowser({
         <div className={matStyles.side}>
           <Input value={query} onChange={setQuery} placeholder={t('materials.search')} />
           {mode === 'editor' && onAdd ? (
-            <Button variant="primary" onClick={onAdd}>
-              {t('materials.add')}
-            </Button>
+            <div className={matStyles.sideAddBtn}>
+              <Button variant="primary" onClick={onAdd}>
+                {t('materials.add')}
+              </Button>
+            </div>
           ) : null}
           <div className={matStyles.list}>
             {filtered.map((m) => (