feat: persist editor state using localStorage (offline draft support)

[amritamishra01]

This PR implements automatic persistence of the editor content
using browser localStorage.

Features:

• Saves editor state locally (debounced 500ms)
• Restores draft on refresh or browser restart
• Respects existing URL/Gist priority logic
• SSR-safe for Next.js
• Uses versioned storage key (lfortran_user_code_v1)
• Minimal diff (logic kept inside pages/index.js)

This enables offline draft support without requiring
GitHub login or internet access.
closes #115

[amritamishra01]

Hi @certik ,

This is the working video :

pr.130.mp4

The video demonstrates the following scenarios:

Draft Persistence – Editor content is saved automatically (debounced) and restored on refresh.
Hard Refresh Support – Draft persists even after Ctrl + Shift + R.
Browser Restart Recovery – Draft restores after closing and reopening the browser.
URL Override Protection – ?code= parameter loads correctly without overwriting the saved draft.
Priority Restoration – Removing the URL parameter restores the previously saved local draft.
Fallback Behavior – When no draft exists, the default example program loads correctly.

This confirms correct priority handling and offline draft persistence behavior.
Thankyou.