docs: update all documentation for Square payment integration
- README: add Square payments to features, expand project structure, update test count
- Overview: mark Square as active, add payment integration + saved cards sections
- Technical Manual: add payment handlers, API endpoints, 4 new DB tables, build tags
- Admin Manual: update Take Payment from stub to live Square Terminal flow
- User Manual: add paying deposits online + leaving tips sections
- Future Work: mark E1-E4 (Square items) as complete
popertots
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