fix(scheduling): rescue candidates should be close-to-qualifying, not zero-mileage

For rescue_hopeless (换走) scenario, completely rethought candidate logic:

Before: showed biggest-gap candidates (0 mileage) → pointless, customer can't
  drive them to target
After: prioritize candidates where customer's remaining driving can push them
  over the target line (canQualifyAfterSwap), sorted by smallest gap first

Example: customer drives 178km/day × 57 days = ~1万km remaining.
- 粤AGR6869 (缺口 1990km) → 换后 3.8万, 可达标 ✅ (shown first)
- 浙FF58720 (缺口 6万km) → 换后 1万, 远不达标 (no longer shown first)

Also updated reason text to explain the math:
"该客户剩余57天还能跑约1万km，足以帮缺口小的车冲线"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/modules/scheduling/SuggestionDetail.tsx

@@ -121,7 +121,7 @@ export default function SuggestionDetail({ suggestion: s, onClose, onNotifySucce
             </div>
             <div className="text-[10px] text-slate-400 mb-2.5">
               {isRescue
-                ? '以下车辆里程已充足，可调给当前客户，将此车换走给高里程客户冲刺'
+                ? '以下车辆快达标，换到当前客户处利用剩余天数即可冲线'
                 : '以下车辆里程缺口大，换到该高里程客户处可加速达标'
               }
             </div>