fix: restore difficulty extraction in content script

[VarshithReddy2006]

Description

This PR restores difficulty extraction in the content script before the blog generation payload is sent.

The payload currently includes the difficulty field:

payload: {
  title,
  description,
  code,
  author,
  client_time,
  custom_prompt,
  difficulty,
  language
}

However, the difficulty extraction logic was missing, causing difficulty to be undefined at runtime.

This change restores the difficulty detection logic and ensures a valid difficulty value is included in generated blog requests.

Related Issue

Fixes #197

Type of Change

• Bug fix
• New feature
• Documentation update

Checklist

• My code follows project guidelines
• I have tested my changes
• Documentation updated if needed

Verification

• Restored difficulty extraction using existing LeetCode difficulty selectors.
• Verified difficulty is defined before being used in the payload.
• Verified the content script passes syntax validation:

node --check extension/content.js

• Confirmed only a single difficulty extraction block exists.

[VarshithReddy2006]

Hi @vanshaggarwal27 ,

While working on #197, I noticed that after removing the duplicate declarations, the content script still references difficulty inside the GENERATE_BLOG payload, but the difficulty extraction logic is no longer present in the current code.

This PR restores the missing difficulty extraction block so that difficulty is defined before being used in the payload, preventing runtime failures and ensuring difficulty information is included in generated blog content.

Could you please take a look when you get a chance? Thank you!