[PROMO-008] Promotions: Flash Sale & Deals landing page

## Overview
Build the Deals landing page that surfaces active flash sales and promotions. Currently the "Browse Deals" button shows a stub — this issue wires it up to live data.

## Acceptance Criteria
- [ ] Route: `/deals` renders active promotions fetched from `GET /api/promotions/active`
- [ ] Each deal card shows: description, discount amount/%, affected products, expiry countdown timer
- [ ] Products on sale show updated prices and a sale badge on the product grid
- [ ] "Add to Cart" on deal items automatically applies the discount
- [ ] Empty state when no active deals are running
- [ ] Meta tags for SEO (deal pages are high-traffic)

## ⚠️ Status: Blocked
This issue is **blocked by #11 PROMO-007** — the discount engine must be built before this page has data to display. If PROMO-007 slips from Sprint 3, this ticket must also be deferred.

## Depends On
- #11 PROMO-007 — discount rules engine provides the data