feat(webui): add limit bars, forecasts, events, exports and URL state

The Usage tab shows a bar for each configured limit. It turns amber at
80% and red at 100%. The tab also shows rejected requests per day, a
projected month-end cost with budget headroom, and CSV export for users
and groups. Names link to Charts with the matching scope.

The Charts tab ranks models over the full 24h, 30d or 12mo window. A
provider filter is applied on the server, and the ranking can be
exported as CSV. A new Events tab lists recent failures, filtered by
kind and user. The selected tab, filters and scope live in the URL
hash. Polling stops while the page is hidden. Views show which replica
answered, and a banner lists configuration warnings.

CSV cells that start with a formula character are neutralised. Links
sit outside the accordion buttons, so both controls keep their
accessible names. admin_assets_gen.go is regenerated.

Claude-Session: https://claude.ai/code/session_01V5jYJNxpDFdJkyyExpADrv

webui/src/components/UsageTable.vue

@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import type { SortingState } from '@tanstack/vue-table'
 import { computed } from 'vue'
 
 import DataTable from '@/components/DataTable.vue'
@@ -21,9 +22,15 @@ const props = defineProps<{
   secondaryValue: (entry: AdminUsageEntryView) => string
   /** Shown in the empty-state row when entries is empty — e.g. a filtered-search "no X match ..." message. Falls through to DataTable's own 'none' default when omitted. */
   emptyMessage?: string
+  /** P1: the shared reactive clock (composables/useNow.ts) — threaded into usageColumns() below so this table's own costMonthProj column recomputes as the clock ticks, instead of freezing at whatever instant this computed first ran. */
+  now: Date
+  /** P6: optional controlled sort state, forwarded to DataTable's own `v-model:sorting` (see that component's identical prop doc comment) — omitted everywhere except UsageView.vue's top-level Users table, whose export must reflect the table's current sort. */
+  sorting?: SortingState
 }>()
 
-const columns = computed(() => usageColumns(props.idLabel, props.secondaryColumnLabel, props.secondaryValue))
+const emit = defineEmits<{ 'update:sorting': [value: SortingState] }>()
+
+const columns = computed(() => usageColumns(props.idLabel, props.secondaryColumnLabel, props.secondaryValue, props.now))
 </script>
 
 <template>
@@ -32,5 +39,7 @@ const columns = computed(() => usageColumns(props.idLabel, props.secondaryColumn
     :data="entries"
     :empty-message="emptyMessage"
     :row-class="(entry) => (entry.storeDown ? 'bg-destructive/10' : undefined)"
+    :sorting="sorting"
+    @update:sorting="(v) => emit('update:sorting', v)"
   />
 </template>