refactor(customize): add keyboard shortcut (Ctrl+K / Cmd+K) to focus the username input (JhaSourav07#474)

## Description

Fixes JhaSourav07#386

## Pillar

- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview
No visual change. When we press Ctrl+K / Cmd+K on the Customization
Studio page and the username input receives focus instantly.

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [x] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

app/customize/page.tsx

@@ -39,8 +39,20 @@ export default function CustomizePage(): ReactElement {
     };
   }, []);
 
-  // Clear custom hex overrides when switching to auto — fixed colors
-  // conflict with the dual-palette prefers-color-scheme switching.
+  useEffect(() => {
+    const handleKeyDown = (event: KeyboardEvent) => {
+      if ((event.ctrlKey || event.metaKey) && event.key.toLowerCase() === 'k') {
+        const input = document.querySelector<HTMLInputElement>('#username-input');
+        if (!input || document.activeElement === input) return;
+        event.preventDefault();
+        input.focus();
+      }
+    };
+
+    window.addEventListener('keydown', handleKeyDown);
+    return () => window.removeEventListener('keydown', handleKeyDown);
+  }, []);
+
   // Clear custom hex overrides when switching to virtual themes because
   // fixed colors conflict with their palette-selection behavior.
   const handleThemeChange = useCallback((newTheme: string): void => {