fix(usage): use real reset header for session window instead of prediction

The 5h window reset time displayed for Setup Token accounts was inaccurate
because UpdateSessionWindow predicted the window end as "current hour + 5h"
instead of reading the actual `anthropic-ratelimit-unified-5h-reset` response
header. This caused the countdown to differ from the official Claude page.

Backend: parse the reset header (Unix timestamp) and use it as the real
window end, falling back to the hour-truncated prediction only when the
header is absent. Also correct stale predictions when a subsequent request
provides the real reset time.

Frontend: add a reactive 60s timer so the reset countdown in
UsageProgressBar ticks down in real-time instead of freezing at the
initial value.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/account/UsageProgressBar.vue

@@ -56,7 +56,8 @@
 </template>
 
 <script setup lang="ts">
-import { computed } from 'vue'
+import { computed, ref } from 'vue'
+import { useIntervalFn } from '@vueuse/core'
 import { useI18n } from 'vue-i18n'
 import type { WindowStats } from '@/types'
 import { formatCompactNumber } from '@/utils/format'
@@ -71,6 +72,12 @@ const props = defineProps<{
 
 const { t } = useI18n()
 
+// Reactive clock for countdown (updates every 60s)
+const now = ref(new Date())
+useIntervalFn(() => {
+  now.value = new Date()
+}, 60_000)
+
 // Label background colors
 const labelClass = computed(() => {
   const colors = {
@@ -119,8 +126,7 @@ const displayPercent = computed(() => {
 const formatResetTime = computed(() => {
   if (!props.resetsAt) return '-'
   const date = new Date(props.resetsAt)
-  const now = new Date()
-  const diffMs = date.getTime() - now.getTime()
+  const diffMs = date.getTime() - now.value.getTime()
 
   if (diffMs <= 0) return '现在'