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  filters out group IDs no longer active.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/api/channels.ts

@@ -0,0 +1,60 @@
+/**
+ * User Channels API endpoints (non-admin)
+ * 用户侧「可用渠道」聚合查询：渠道 + 用户可访问的分组 + 支持模型（含定价）。
+ */
+
+import { apiClient } from './client'
+import type { BillingMode } from '@/constants/channel'
+
+export interface UserAvailableGroup {
+  id: number
+  name: string
+  platform: string
+}
+
+export interface UserPricingInterval {
+  min_tokens: number
+  max_tokens: number | null
+  tier_label?: string
+  input_price: number | null
+  output_price: number | null
+  cache_write_price: number | null
+  cache_read_price: number | null
+  per_request_price: number | null
+}
+
+export interface UserSupportedModelPricing {
+  billing_mode: BillingMode
+  input_price: number | null
+  output_price: number | null
+  cache_write_price: number | null
+  cache_read_price: number | null
+  image_output_price: number | null
+  per_request_price: number | null
+  intervals: UserPricingInterval[]
+}
+
+export interface UserSupportedModel {
+  name: string
+  platform: string
+  pricing: UserSupportedModelPricing | null
+}
+
+export interface UserAvailableChannel {
+  name: string
+  description: string
+  groups: UserAvailableGroup[]
+  supported_models: UserSupportedModel[]
+}
+
+/** 列出当前用户可见的「可用渠道」（与 /groups/available 保持一致，返回平数组）。 */
+export async function getAvailable(options?: { signal?: AbortSignal }): Promise<UserAvailableChannel[]> {
+  const { data } = await apiClient.get<UserAvailableChannel[]>('/channels/available', {
+    signal: options?.signal
+  })
+  return data
+}
+
+export const userChannelsAPI = { getAvailable }
+
+export default userChannelsAPI