Show prices in pollen instead of $ + flip to local currency instead of $

[ElliotEtag]

• Reverse price display: lead with "10 pollen" as the primary unit, local-currency equivalent below (e.g. "≈ $9 / €8.30"). 1 pollen = $1; local currency is presentational. Today the UI leads with USD and reveals pollen quantity as the bonus
• Flip pricing model from bonus-pollen to discount-USD: today "$5 → 6 pollen", change to "$4.50 → 5 pollen" (10% off). Re-pick clean tiered discounts per pack (e.g. p5: 10%, p10: 20%, p20: 25%, p50: 28%, p100: 30%) rather than mirroring current bonus math. Touches shared/pollen-packs.ts, pollen-balance.tsx, Stripe price_data.unit_amount
• Related: feat: country-routed checkout (phase 1 backend) #11172