fix: prevent navbar from appearing above modals (JhaSourav07#3419)

## Description

Fixes JhaSourav07#2128

This PR fixes a UI layering issue where the navbar remained visible
above modal overlays.

### Root Cause

The navbar component had a global `z-50` class applied, causing it to
render above modal backdrops and dialogs across the application.

### Changes Made

* Removed the unnecessary `z-50` class from the navbar component.
* Ensured modal overlays correctly cover the navbar when a modal is
opened.
* Verified the fix across multiple modals to ensure consistent behavior
throughout the application.

### Files Changed

* `app/components/navbar.tsx`

## Pillar

* [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview

### Before

The navbar remained visible above modal overlays, reducing focus on the
active dialog.

(Add before screenshot)

<img width="1920" height="1200" alt="image"
src="https://github.com/user-attachments/assets/92466f46-5b5b-4bb5-99aa-c6c7f557dad2"
/>

The navbar is properly covered by the modal overlay, providing a cleaner
and more consistent modal experience.

<img width="1920" height="1200" alt="image"
src="https://github.com/user-attachments/assets/52ebfb2d-41bb-4a49-aa1f-b965f6b6fd79"
/>

## 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): ...`).
* [ ] I have updated `README.md` if I added a new theme or URL
parameter. (Not applicable)
* [x] I have starred 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 SVG changes were made).
* [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

app/components/navbar.tsx

@@ -74,7 +74,7 @@ export default function Navbar() {
   };
 
   return (
-    <header className="relative z-50 px-4 pt-4 sm:px-6 w-full">
+    <header className="relative px-4 pt-4 sm:px-6 w-full">
       <div className="mx-auto max-w-6xl">
         <div
           ref={shellRef}