feat(channels): add "Available Channels" aggregate view

Add a read-only aggregate view per channel: its linked groups and a
deterministic wildcard-free supported-model list with pricing details.

Backend
- service.Channel.SupportedModels(): combine ModelMapping keys with
  same-platform ModelPricing.Models; trailing "*" keys expand via
  pricing prefix match; platforms without a mapping produce no
  entries (intentional "no mapping = not shown" rule).
- Extract splitWildcardSuffix() shared with toModelEntry.
- Build a per-call pricing lookup map (platform+lowerName -> *pricing)
  to avoid O(N*M) scans in SupportedModels.
- ChannelService.ListAvailable() aggregates channels + active groups;
  filters out group IDs no longer active.
- Admin route GET /api/v1/admin/channels/available returns the full
  DTO (id, status, billing_model_source, restrict_models, groups,
  supported_models).
- User route GET /api/v1/channels/available applies three filters:
  Status==active, visible-group intersection, and platform filter
  on supported_models (prevents cross-platform leak when a channel
  links to both a user-accessible group and an inaccessible one on
  another platform). Response is a plain array (matches the
  /groups/available sibling shape). Field whitelist omits
  billing_model_source, restrict_models, ids, status, sort_order.

Frontend
- New /admin/available-channels and /available-channels views backed
  by a shared AvailableChannelsTable component (admin adds status +
  billing-source columns via slots).
- PricingRow extracted to its own SFC; SupportedModelChip references
  shared billing-mode constants in constants/channel.ts.
- Sidebar: new entry above "渠道管理" for admin; matching entry in
  user nav.
- i18n: zh + en coverage for both namespaces.

Tests
- SupportedModels: wildcard-only pricing skipped, prefix-matches-
  nothing, cross-platform bleed, case-insensitive dedup, empty
  platform mapping.
- ListAvailable: nil groupRepo, inactive-group-ID dropped, stable
  case-insensitive name sort.
- User handler: 401 on unauthenticated, visible-group intersection,
  platform filter on supported_models, JSON whitelist.
- Admin handler: full DTO including default BillingModelSource
  fallback.

Refs: issue #1729

frontend/src/components/channels/AvailableChannelsTable.vue

@@ -0,0 +1,110 @@
+<template>
+  <DataTable :columns="columns" :data="rows" :loading="loading">
+    <template #cell-name="{ row }">
+      <div class="font-medium text-gray-900 dark:text-white">{{ row.name }}</div>
+      <div
+        v-if="row.description"
+        class="mt-0.5 text-xs text-gray-500 dark:text-gray-400"
+      >
+        {{ row.description }}
+      </div>
+    </template>
+
+    <template #cell-groups="{ row }">
+      <div v-if="row.groups.length === 0" class="text-xs text-gray-400">
+        <slot name="empty-groups">-</slot>
+      </div>
+      <div v-else class="flex flex-wrap gap-1">
+        <span
+          v-for="g in row.groups"
+          :key="g.id"
+          class="inline-flex items-center rounded bg-blue-50 px-2 py-0.5 text-xs font-medium text-blue-700 dark:bg-blue-900/30 dark:text-blue-300"
+        >
+          {{ g.name }}
+        </span>
+      </div>
+    </template>
+
+    <template #cell-supported_models="{ row }">
+      <div v-if="row.supported_models.length === 0" class="text-xs text-gray-400">
+        {{ noModelsLabel }}
+      </div>
+      <div v-else class="flex max-w-[560px] flex-wrap gap-1">
+        <SupportedModelChip
+          v-for="m in row.supported_models"
+          :key="`${m.platform}-${m.name}`"
+          :model="m"
+          :pricing-key-prefix="pricingKeyPrefix"
+          :no-pricing-label="noPricingLabel"
+        />
+      </div>
+    </template>
+
+    <!-- 允许父组件为额外列提供自定义渲染（如 admin 的 status / billing_model_source）。 -->
+    <template v-for="slot in extraCellSlots" :key="slot" #[slot]="scope">
+      <slot :name="slot" v-bind="scope" />
+    </template>
+
+    <template #empty>
+      <slot name="empty">
+        <div class="flex flex-col items-center py-8">
+          <Icon name="inbox" size="xl" class="mb-3 h-12 w-12 text-gray-400" />
+          <p class="text-sm text-gray-500 dark:text-gray-400">{{ emptyLabel }}</p>
+        </div>
+      </slot>
+    </template>
+  </DataTable>
+</template>
+
+<script setup lang="ts">
+import { computed, useSlots } from 'vue'
+import DataTable from '@/components/common/DataTable.vue'
+import Icon from '@/components/icons/Icon.vue'
+import SupportedModelChip from './SupportedModelChip.vue'
+
+interface GroupRef {
+  id: number
+  name: string
+  platform?: string
+}
+
+interface Row {
+  name: string
+  description?: string
+  groups: GroupRef[]
+  supported_models: Array<{
+    name: string
+    platform: string
+    pricing: unknown | null
+  }>
+  [key: string]: unknown
+}
+
+interface Column {
+  key: string
+  label: string
+}
+
+withDefaults(
+  defineProps<{
+    columns: Column[]
+    rows: Row[]
+    loading: boolean
+    pricingKeyPrefix: string
+    noPricingLabel: string
+    noModelsLabel: string
+    emptyLabel: string
+  }>(),
+  { loading: false }
+)
+
+const slots = useSlots()
+/**
+ * 透传父组件提供的 cell-* 插槽（除本组件内置的 name/groups/supported_models/empty-groups/empty
+ * 之外），让 admin 场景可以自定义 status / billing_model_source 等列。
+ */
+const extraCellSlots = computed(() => {
+  const reserved = new Set(['cell-name', 'cell-groups', 'cell-supported_models', 'empty-groups', 'empty'])
+  return Object.keys(slots).filter((name) => name.startsWith('cell-') && !reserved.has(name))
+})
+</script>