feat: add comment toggle with Ctrl+/

[Jatin-Shihora]

There's no quick way to comment or uncomment lines of code. You have to manually type // or # at the start of each line. When debugging, you often want to quickly comment out a block of code and uncomment it later.

What should happen

• Ctrl+/ toggles a comment on the current line
• If there's an active selection, it toggles comments on all selected lines
• If the line is already commented, it removes the comment prefix. If not, it adds one
• The comment prefix should match the current language (// for C++/Java/JS/Go/Rust, # for Python/Ruby, -- for SQL, etc.)

Things to consider

• The current language is tracked in problem_detail.py and passed to the editor. You'll need the language info to determine the correct comment syntax
• Comment prefixes per language are straightforward. Add a mapping in constants.py next to the existing LANG_SLUGS and LANG_EXTENSIONS maps
• When toggling on multiple lines, check if ALL selected lines are commented before deciding to comment or uncomment. If they're mixed, comment all of them
• This should integrate with undo/redo so Ctrl+U reverses the toggle