Add behavior documentation covering all SSR and legacy SPA pages:
- Testing strategy and type definitions (unit/integration/UI)
- Dashboard, Invoice, Payment, Transaction, Ledger pages
- Company/Settings, POS, Admin, Search, Auth pages
- Legacy SPA behavior docs (no UI tests until migrated)
- Edge cases, test data requirements, and dependencies per subsystem
Total: 3,600+ lines of behavior documentation to guide test authorship.
