Extract magic string constants

[malston]

Summary

Category names and URL patterns are hardcoded throughout categorizeUrl() and other functions.

Current Behavior

Magic strings scattered throughout code:

• Category names: "Social Media", "Shopping", "News", etc.
• URL patterns for categorization
• Group name formats

Expected Behavior

Extract to named constants in a configuration module for easier maintenance and consistency.

Priority

Low

Related

Split from #3 (item #11)