Add tip categories or tags for organizing tips by purpose

[Mosas2000]

Description

All tips are stored in the same undifferentiated stream. Adding categories (e.g., 'content creation', 'open source', 'community help', 'appreciation') would enable filtering, analytics, and richer leaderboards.

Suggested Fix

Add an optional category field to the tip data, either as a string or a uint enum. Define a set of standard categories in the contract. Allow the frontend to filter the tip feed by category. This feature would also enable category-specific leaderboards and analytics dashboards.