Add Diff Checker tool

[GhDj]

Summary

• Add new Diff Checker tool for comparing texts side-by-side
• Uses LCS (Longest Common Subsequence) algorithm for accurate diffs
• Visual highlighting of additions and deletions

Features

• Side-by-side View: Original and modified text shown in parallel
• Line Highlighting: Green for additions, red for deletions
• Line Numbers: Shows line numbers for easy reference
• Statistics: Count of added, removed, and unchanged lines
• Ignore Options: Toggle whitespace and case sensitivity
• Swap Function: Quickly swap original and modified texts
• Sample Code: Load example to test the tool

Test plan

• Verify diff tool page loads at /tools/diff
• Test comparison with sample code
• Verify highlighting works correctly
• Test ignore whitespace/case options
• All 273 tests pass

[GhDj]

Merged via cherry-pick into develop