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  filters out group IDs no longer active.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

@@ -163,5 +163,42 @@ export async function getModelDefaultPricing(model: string): Promise<ModelDefaul
   return data
 }
 
-const channelsAPI = { list, getById, create, update, remove, getModelDefaultPricing }
+// --- Available channels (聚合视图：渠道 + 分组 + 支持模型) ---
+
+export interface AvailableGroupRef {
+  id: number
+  name: string
+  platform: string
+}
+
+export interface SupportedModel {
+  name: string
+  platform: string
+  pricing: ChannelModelPricing | null
+}
+
+export interface AvailableChannel {
+  id: number
+  name: string
+  description: string
+  status: string
+  billing_model_source: string
+  restrict_models: boolean
+  groups: AvailableGroupRef[]
+  supported_models: SupportedModel[]
+}
+
+interface AvailableChannelsResponse {
+  items: AvailableChannel[]
+}
+
+/** 列出所有可用渠道（含关联分组与支持模型） */
+export async function listAvailable(options?: { signal?: AbortSignal }): Promise<AvailableChannel[]> {
+  const { data } = await apiClient.get<AvailableChannelsResponse>('/admin/channels/available', {
+    signal: options?.signal
+  })
+  return data.items
+}
+
+const channelsAPI = { list, getById, create, update, remove, getModelDefaultPricing, listAvailable }
 export default channelsAPI