feat(frontend): Dashboard 精修(切车器 + 累计升数 + 错误态)

把 Dashboard 从基本骨架升级到 MVP 可用状态:
- 切车:多车辆时显示 NSelect,直接切;单车时只显示名字(不让选)
  * 切车 watch currentId 自动重拉 statistics,无需手动刷新
- 主指标卡:把"本月加油"标题改成"累计加油",因为目前后端没给"本月"
  过滤,只是总量;数字含义与 Backend RefuelStatistics.total_amount 一致
- 4 项指标:平均油价 / 平均油耗 / 每公里成本 / 累计升数 —— 之前只有 3 项,
  补上总升数
- Empty / Loading / Error 三态完整:
  * Loading -> NSpin
  * 没记录但有车 -> "还没有加油记录"
  * API 失败 -> useMessage.error + stats 清空
- 保留已有:大字号主指标、"记录加油"主按钮、"最近加油"卡片

未做(留待后续):
- "本月"维度过滤(后端可加 query?period=month,Dashboard 切 month/all)
  —— plan §9.1 提了"本月加油",MVP 留总量已经够
- 离线缓存 / PWA install prompt

验证:
  pnpm typecheck   0 errors
  pnpm build       通过

fuellog-web/src/pages/Dashboard.vue

@@ -1,20 +1,36 @@
 <script setup lang="ts">
-import { onMounted, ref } from 'vue'
+import { computed, onMounted, ref, watch } from 'vue'
-import { NButton, NCard, NSpin, NTag, NEmpty, NText } from 'naive-ui'
+import {
+  NButton,
+  NCard,
+  NEmpty,
+  NSpin,
+  NTag,
+  NText,
+  NSelect,
+  useMessage,
+} from 'naive-ui'
 import { useVehicleStore } from '@/stores/vehicle'
 import { refuelsApi } from '@/api/refuels'
 import type { RefuelStatistics } from '@/api/types'
 import { fmtAmount, fmtConsumption, fmtCostPerKm, fmtPrice } from '@/utils/decimal'
 
 const vehicleStore = useVehicleStore()
+const message = useMessage()
 const stats = ref<RefuelStatistics | null>(null)
 const loading = ref(false)
 
 async function loadStats() {
-  if (!vehicleStore.currentId) return
+  if (!vehicleStore.currentId) {
+    stats.value = null
+    return
+  }
   loading.value = true
   try {
     stats.value = await refuelsApi.statistics(vehicleStore.currentId)
+  } catch (err) {
+    message.error((err as Error).message || '加载统计失败')
+    stats.value = null
   } finally {
     loading.value = false
   }
@@ -24,34 +40,65 @@ onMounted(async () => {
   await vehicleStore.loadList()
   await loadStats()
 })
+
+watch(
+  () => vehicleStore.currentId,
+  () => loadStats(),
+)
+
+const currentVehicle = computed(() => vehicleStore.getCurrent())
+
+const vehicleOptions = computed(() =>
+  vehicleStore.list.map((v) => ({
+    label: `${v.name}${v.default_vehicle ? ' (默认)' : ''}`,
+    value: v.id,
+  })),
+)
+
+function onVehicleChange(id: number | null) {
+  if (id != null) vehicleStore.setCurrentId(id)
+}
 </script>
 
 <template>
   <div class="space-y-4 max-w-screen-md mx-auto">
     <NCard v-if="!vehicleStore.list.length" class="text-center py-12">
       <NEmpty description="还没有车辆,先去添加一辆吧">
-        <NButton type="primary" @click="$router.push({ name: 'vehicles' })">添加车辆</NButton>
+        <NButton type="primary" @click="$router.push({ name: 'vehicles' })">
+          添加车辆
+        </NButton>
       </NEmpty>
     </NCard>
 
     <template v-else>
-      <div>
+      <!-- 切车器:多车时显示,单车时只显示名字(不让选) -->
+      <div v-if="vehicleStore.list.length > 1" class="flex items-center gap-2">
+        <NText depth="3" class="text-sm">当前车辆</NText>
+        <NSelect
+          :value="vehicleStore.currentId"
+          :options="vehicleOptions"
+          size="small"
+          style="min-width: 160px"
+          @update:value="onVehicleChange"
+        />
+      </div>
+      <div v-else>
         <NText depth="3" class="text-sm">当前车辆</NText>
         <div class="text-2xl font-semibold mt-1">
-          {{ vehicleStore.getCurrent()?.name || '—' }}
+          {{ currentVehicle?.name || '—' }}
         </div>
       </div>
 
       <NSpin :show="loading">
         <NCard v-if="stats" class="text-center py-6">
-          <div class="text-xs text-ink-soft">本月加油</div>
+          <div class="text-xs text-ink-soft">累计加油</div>
           <div class="text-5xl font-bold mt-1 tracking-tight">
             {{ fmtAmount(stats.total_amount) }}
           </div>
           <div class="text-sm text-ink-soft mt-1">
             共 {{ stats.total_records }} 笔
           </div>
-          <div class="grid grid-cols-2 gap-4 mt-6">
+          <div class="grid grid-cols-2 gap-4 mt-6 text-left">
             <div>
               <div class="text-xs text-ink-soft">平均油价</div>
               <div class="text-lg font-semibold mt-0.5">
@@ -64,14 +111,21 @@ onMounted(async () => {
                 {{ fmtConsumption(stats.avg_consumption) }}
               </div>
             </div>
-            <div class="col-span-2">
+            <div>
               <div class="text-xs text-ink-soft">每公里成本</div>
               <div class="text-lg font-semibold mt-0.5">
                 {{ fmtCostPerKm(stats.cost_per_km) }}
               </div>
             </div>
+            <div>
+              <div class="text-xs text-ink-soft">累计升数</div>
+              <div class="text-lg font-semibold mt-0.5">
+                {{ stats.total_liters.toFixed(2) }} L
+              </div>
+            </div>
           </div>
         </NCard>
+        <NEmpty v-else-if="!loading" description="还没有加油记录" />
       </NSpin>
 
       <NButton