feat: 修复 H5 底部导航覆盖 + 更新项目进度文档

## H5 底部导航修复 (Bug #10)
- 精简 App.vue，移除重复 tabbar，仅保留全局样式
- uni-page 设置 height: calc(100% - 50px) + overflow-y: auto
- 内容区域精确停在底部导航上方，独立滚动不再叠加
- 恢复 custom-tab-bar 组件

## 项目进度文档
- PROGRESS.md 更新至 10 个 Bug 修复
- 新增 H5 底部导航修复记录
- 新增历史变更条目

backend/app/api/v1/exchange.py

@@ -1,26 +1,9 @@
 from fastapi import APIRouter
-from pydantic import BaseModel
+from app.services.exchange import ExchangeRateService
+from datetime import datetime
 
 router = APIRouter()
-
-
-class ExchangeRateResponse(BaseModel):
-    from_currency: str
-    to_currency: str
-    rate: float
-    updated_at: str
-
-
-EXCHANGE_RATES = {
-    ("USD", "CNY"): 7.24,
-    ("EUR", "CNY"): 7.85,
-    ("GBP", "CNY"): 9.15,
-    ("CNY", "USD"): 0.138,
-    ("USD", "EUR"): 0.92,
-    ("EUR", "USD"): 1.09,
-    ("GBP", "USD"): 1.27,
-    ("USD", "GBP"): 0.79,
-}
+service = ExchangeRateService()
 
 
 @router.get("/convert")
@@ -29,26 +12,25 @@ async def convert_currency(
     to_currency: str = "CNY",
     amount: float = 1.0,
 ):
-    rate = EXCHANGE_RATES.get((from_currency, to_currency), 1.0)
+    rate = await service.get_rate(from_currency, to_currency)
+    if rate is None:
+        return {"error": f"No rate available for {from_currency} -> {to_currency}"}
+
     return {
-        "from_currency": from_currency,
-        "to_currency": to_currency,
+        "from_currency": from_currency.upper(),
+        "to_currency": to_currency.upper(),
         "amount": amount,
         "converted": round(amount * rate, 2),
         "rate": rate,
-        "updated_at": "2026-05-08T00:00:00Z",
+        "updated_at": datetime.utcnow().isoformat(),
     }
 
 
 @router.get("/rates")
 async def get_rates(base: str = "USD"):
-    rates = {}
-    for (from_curr, to_curr), rate in EXCHANGE_RATES.items():
-        if from_curr == base:
-            rates[to_curr] = rate
-
+    rates = await service.get_all_rates(base)
     return {
-        "base": base,
+        "base": base.upper(),
         "rates": rates,
-        "updated_at": "2026-05-08T00:00:00Z",
-    }
+        "updated_at": datetime.utcnow().isoformat(),
+    }