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  filters out group IDs no longer active.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/views/user/AvailableChannelsView.vue

@@ -0,0 +1,98 @@
+<template>
+  <AppLayout>
+    <TablePageLayout>
+      <template #filters>
+        <div class="flex flex-col justify-between gap-4 lg:flex-row lg:items-start">
+          <div class="flex flex-1 flex-wrap items-center gap-3">
+            <div class="relative w-full sm:w-80">
+              <Icon
+                name="search"
+                size="md"
+                class="absolute left-3 top-1/2 -translate-y-1/2 text-gray-400 dark:text-gray-500"
+              />
+              <input
+                v-model="searchQuery"
+                type="text"
+                :placeholder="t('availableChannels.searchPlaceholder')"
+                class="input pl-10"
+              />
+            </div>
+          </div>
+
+          <div class="flex w-full flex-shrink-0 flex-wrap items-center justify-end gap-3 lg:w-auto">
+            <button
+              @click="loadChannels"
+              :disabled="loading"
+              class="btn btn-secondary"
+              :title="t('common.refresh', 'Refresh')"
+            >
+              <Icon name="refresh" size="md" :class="loading ? 'animate-spin' : ''" />
+            </button>
+          </div>
+        </div>
+      </template>
+
+      <template #table>
+        <AvailableChannelsTable
+          :columns="columns"
+          :rows="filteredChannels"
+          :loading="loading"
+          pricing-key-prefix="availableChannels.pricing"
+          :no-pricing-label="t('availableChannels.noPricing')"
+          :no-models-label="t('availableChannels.noModels')"
+          :empty-label="t('availableChannels.empty')"
+        />
+      </template>
+    </TablePageLayout>
+  </AppLayout>
+</template>
+
+<script setup lang="ts">
+import { computed, onMounted, ref } from 'vue'
+import { useI18n } from 'vue-i18n'
+import AppLayout from '@/components/layout/AppLayout.vue'
+import TablePageLayout from '@/components/layout/TablePageLayout.vue'
+import Icon from '@/components/icons/Icon.vue'
+import AvailableChannelsTable from '@/components/channels/AvailableChannelsTable.vue'
+import userChannelsAPI, { type UserAvailableChannel } from '@/api/channels'
+import { useAppStore } from '@/stores/app'
+import { extractApiErrorMessage } from '@/utils/apiError'
+
+const { t } = useI18n()
+const appStore = useAppStore()
+
+const channels = ref<UserAvailableChannel[]>([])
+const loading = ref(false)
+const searchQuery = ref('')
+
+const columns = computed(() => [
+  { key: 'name', label: t('availableChannels.columns.name') },
+  { key: 'groups', label: t('availableChannels.columns.groups') },
+  { key: 'supported_models', label: t('availableChannels.columns.supportedModels') }
+])
+
+const filteredChannels = computed(() => {
+  const q = searchQuery.value.trim().toLowerCase()
+  if (!q) return channels.value
+  return channels.value.filter((ch) => {
+    if (ch.name.toLowerCase().includes(q)) return true
+    if ((ch.description || '').toLowerCase().includes(q)) return true
+    if (ch.groups.some((g) => g.name.toLowerCase().includes(q))) return true
+    if (ch.supported_models.some((m) => m.name.toLowerCase().includes(q))) return true
+    return false
+  })
+})
+
+async function loadChannels() {
+  loading.value = true
+  try {
+    channels.value = await userChannelsAPI.getAvailable()
+  } catch (err: unknown) {
+    appStore.showError(extractApiErrorMessage(err, t('common.error')))
+  } finally {
+    loading.value = false
+  }
+}
+
+onMounted(loadChannels)
+</script>