feat(pos): rebuild Sales — 3 tabs, smooth checkout + tender/change, print+email receipts
7 tabs → Register / History / Settings. Register: search/scan + cart + checkout flow
(cash tender→change, split tenders, layby deposit, notes) → completes → receipt auto-pops.
Receipts now server-rendered (app/receipts.py, one template) with per-item discounts, GST
breakdown, split lines, tendered/change, store header/footer; Print opens a clean window
(no visibility hack); Email via app/mailer.py (stdlib SMTP, creds in vault, admin-only) with
a Settings 'send test' button. History merges past sales + laybys (reprint/collect). Settings
holds currency/GST/discount + receipt header/footer + promotions. sales gains amount_tendered
+ notes; SaleIn carries tendered/notes; /sales/{id}/receipt + /{id}/email + /email-test added.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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