Add proper empty state before username input

[paanipoorie]

Description

Before entering a GitHub username, the preview area shows a broken or blank state (e.g., "Preview" text or an empty box).

This creates confusion and makes the UI feel incomplete.

Steps to Reproduce

1. Open the app
2. Do not enter any username
3. Observe the preview section

Current Behavior

• Preview area is empty or shows a broken placeholder ("Preview")
• No guidance is provided to the user

Expected Behavior

• Display a clear placeholder UI or message:

  "Enter a GitHub username to preview"

• Maintain visual consistency with the rest of the design

Acceptance Criteria

• Add a proper empty state UI before input
• Remove broken or placeholder text ("Preview")
• Provide clear guidance message to the user
• Ensure styling matches the existing design system

Scope

• Frontend/UI only
• Do not modify API logic

Screenshots

[ManthanTerse]

Hi, I'd like to work on this under NSOC'26! Could you please assign it to me?
@paanipoorie @JhaSourav07

[paanipoorie]

Hi, I'd like to work on this under NSOC'26! Could you please assign it to me?
@paanipoorie @JhaSourav07

Ok , waiting for the PR.