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  filters out group IDs no longer active.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/i18n/locales/zh.ts

@@ -344,6 +344,7 @@ export default {
     users: '用户管理',
     groups: '分组管理',
     channels: '渠道管理',
+    availableChannels: '可用渠道',
     subscriptions: '订阅管理',
     accounts: '账号管理',
     proxies: 'IP管理',
@@ -933,6 +934,38 @@ export default {
     }
   },
 
+  // Available Channels (user-facing)
+  availableChannels: {
+    title: '可用渠道',
+    description: '查看您可访问的渠道与其支持的模型、定价',
+    searchPlaceholder: '搜索渠道或模型...',
+    empty: '暂无可用渠道',
+    noModels: '未配置模型',
+    noPricing: '未配置定价',
+    columns: {
+      name: '渠道名',
+      groups: '我可访问的分组',
+      supportedModels: '支持模型'
+    },
+    pricing: {
+      billingMode: '计费模式',
+      billingModeToken: '按 Token',
+      billingModePerRequest: '按次',
+      billingModeImage: '按图片',
+      inputPrice: '输入',
+      outputPrice: '输出',
+      cacheWritePrice: '缓存写入',
+      cacheReadPrice: '缓存读取',
+      imageOutputPrice: '图片输出',
+      perRequestPrice: '每次请求',
+      intervals: '阶梯定价',
+      tierLabel: '层级',
+      tokenRange: 'Token 区间',
+      unitPerMillion: '/ 1M token',
+      unitPerRequest: '/ 次'
+    }
+  },
+
   // Redeem
   redeem: {
     title: '兑换码',
@@ -2059,6 +2092,48 @@ export default {
       }
     },
 
+    // Available Channels (aggregated read-only view)
+    availableChannels: {
+      title: '可用渠道',
+      description: '按渠道聚合查看关联分组与支持模型（已展开通配符）',
+      searchPlaceholder: '搜索渠道或模型...',
+      columns: {
+        name: '渠道名',
+        status: '状态',
+        billingSource: '计费模型来源',
+        groups: '关联分组',
+        supportedModels: '支持模型'
+      },
+      empty: '暂无数据',
+      noGroups: '未关联分组',
+      noModels: '未配置模型映射',
+      noPricing: '未配置定价',
+      statusActive: '启用',
+      statusDisabled: '停用',
+      billingSource: {
+        requested: '请求模型',
+        upstream: '上游模型',
+        channel_mapped: '映射后模型'
+      },
+      pricing: {
+        billingMode: '计费模式',
+        billingModeToken: '按 Token',
+        billingModePerRequest: '按次',
+        billingModeImage: '按图片',
+        inputPrice: '输入',
+        outputPrice: '输出',
+        cacheWritePrice: '缓存写入',
+        cacheReadPrice: '缓存读取',
+        imageOutputPrice: '图片输出',
+        perRequestPrice: '每次请求',
+        intervals: '阶梯定价',
+        tierLabel: '层级',
+        tokenRange: 'Token 区间',
+        unitPerMillion: '/ 1M token',
+        unitPerRequest: '/ 次'
+      }
+    },
+
     // Channel Management
     channels: {
       title: '渠道管理',