[Feature] Add language/syntax highlighting selector

[kom50]

Summary

Add a dropdown selector to switch between different programming languages for syntax highlighting.

Requirements

• Add dropdown in header with language options
• Support languages: JavaScript, TypeScript, Python, HTML, CSS, Markdown, JSON, SQL, Java, C/C++, Go, Rust, PHP, Ruby
• Persist language choice per room (sync to all users)
• Update file extension for download feature
• Store language preference on server

UI/UX

• Dropdown should be styled to match dark/light theme
• Show current language icon/name
• Group languages by category (Web, Systems, Scripting, etc.)

Technical Notes

• CodeMirror modes to load dynamically
• Emit language-change event via Socket.IO
• Server should store room.language property

Acceptance Criteria

• User can switch language and syntax updates immediately
• All users in room see the language change
• New users joining see correct language