Fix price drop notifications

bugfix: ducat comparison shows wrong recommendation due to incorrect thresholds #1: Ducat ratio thresholds raised to 8/5 (was 3/2) — reflects real community value of ~0.1–0.2p/d. Fixes false "Ducats may be worth considering" on items like Voruna Prime (0.34p/d).
bugfix ux: clarify liquidity score tooltip — make 1=low, 10=high explicit #2: Liquidity tooltip now states scale direction explicitly: "1 = very hard to sell, 10 = trades daily".
new feature: show best hour to sell (in addition to best hour to buy) #3: New 💰 sell hour chip — shows best hour to sell based on hourly averages. Reuses the already-computed worst variable in calcBestHour().
new feature: show bid-ask spread on item pages #4: New ↔ bid-ask spread chip — fetches /orders, filters online/ingame users, computes min_sell − max_buy. Color-coded by tightness (green ≤5p, orange >5p, red if negative).