fix(theme): wrong theme application during preview (@byseif21) (monkeytypegame#6617)

### Description

Closes monkeytypegame#6616 

Changes:
- Refactored theme preview state management to use a single state object
- Removed redundant state variables
- Maintained the same debouncing behavior for smooth scrolling

Author: byseif21

frontend/src/ts/controllers/theme-controller.ts

@@ -231,20 +231,27 @@ function updateFooterIndicator(nameOverride?: string): void {
   }
 }
 
+type PreviewState = {
+  theme: string;
+  colors?: string[];
+} | null;
+
+let previewState: PreviewState = null;
+
 export function preview(
   themeIdentifier: string,
   customColorsOverride?: string[]
 ): void {
-  debouncedPreview(themeIdentifier, customColorsOverride);
+  previewState = { theme: themeIdentifier, colors: customColorsOverride };
+  debouncedPreview();
 }
 
-const debouncedPreview = debounce<(t: string, c?: string[]) => void>(
-  250,
-  (themeIdenfitier, customColorsOverride) => {
+const debouncedPreview = debounce<() => void>(250, () => {
+  if (previewState) {
     isPreviewingTheme = true;
-    void apply(themeIdenfitier, customColorsOverride, true);
+    void apply(previewState.theme, previewState.colors, true);
   }
-);
+});
 
 async function set(
   themeIdentifier: string,
@@ -264,6 +271,8 @@ async function set(
 }
 
 export async function clearPreview(applyTheme = true): Promise<void> {
+  previewState = null;
+
   if (isPreviewingTheme) {
     isPreviewingTheme = false;
     if (applyTheme) {