Add achievements page

- Create /dashboard/achievements page with badge gallery
- Define 18 achievements across 4 categories (progress, streak, performance, collection)
- Show unlocked vs locked achievements with visual distinction
- Display rarity levels (common, uncommon, rare, epic, legendary)
- Add category filter and stats summary
- Include tips section for unlocking achievements

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: alfredang