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  filters out group IDs no longer active.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/en.ts

@@ -344,6 +344,7 @@ export default {
     users: 'Users',
     groups: 'Groups',
     channels: 'Channels',
+    availableChannels: 'Available Channels',
     subscriptions: 'Subscriptions',
     accounts: 'Accounts',
     proxies: 'Proxies',
@@ -929,6 +930,38 @@ export default {
     }
   },
 
+  // Available Channels (user-facing)
+  availableChannels: {
+    title: 'Available Channels',
+    description: 'Channels you can access, along with their supported models and pricing',
+    searchPlaceholder: 'Search channels or models...',
+    empty: 'No available channels',
+    noModels: 'No models configured',
+    noPricing: 'Pricing not configured',
+    columns: {
+      name: 'Channel',
+      groups: 'Your Accessible Groups',
+      supportedModels: 'Supported Models'
+    },
+    pricing: {
+      billingMode: 'Billing Mode',
+      billingModeToken: 'Per Token',
+      billingModePerRequest: 'Per Request',
+      billingModeImage: 'Per Image',
+      inputPrice: 'Input',
+      outputPrice: 'Output',
+      cacheWritePrice: 'Cache Write',
+      cacheReadPrice: 'Cache Read',
+      imageOutputPrice: 'Image Output',
+      perRequestPrice: 'Per Request',
+      intervals: 'Tiered Pricing',
+      tierLabel: 'Tier',
+      tokenRange: 'Token Range',
+      unitPerMillion: '/ 1M tokens',
+      unitPerRequest: '/ request'
+    }
+  },
+
   // Redeem
   redeem: {
     title: 'Redeem Code',
@@ -1980,6 +2013,48 @@ export default {
       }
     },
 
+    // Available Channels (aggregated read-only view)
+    availableChannels: {
+      title: 'Available Channels',
+      description: 'Aggregated view: each channel with its linked groups and supported models (wildcards expanded)',
+      searchPlaceholder: 'Search channels or models...',
+      columns: {
+        name: 'Channel',
+        status: 'Status',
+        billingSource: 'Billing Model Source',
+        groups: 'Linked Groups',
+        supportedModels: 'Supported Models'
+      },
+      empty: 'No data',
+      noGroups: 'No linked groups',
+      noModels: 'No model mapping configured',
+      noPricing: 'Pricing not configured',
+      statusActive: 'Active',
+      statusDisabled: 'Disabled',
+      billingSource: {
+        requested: 'Requested model',
+        upstream: 'Upstream model',
+        channel_mapped: 'Channel-mapped model'
+      },
+      pricing: {
+        billingMode: 'Billing Mode',
+        billingModeToken: 'Per Token',
+        billingModePerRequest: 'Per Request',
+        billingModeImage: 'Per Image',
+        inputPrice: 'Input',
+        outputPrice: 'Output',
+        cacheWritePrice: 'Cache Write',
+        cacheReadPrice: 'Cache Read',
+        imageOutputPrice: 'Image Output',
+        perRequestPrice: 'Per Request',
+        intervals: 'Tiered Pricing',
+        tierLabel: 'Tier',
+        tokenRange: 'Token Range',
+        unitPerMillion: '/ 1M tokens',
+        unitPerRequest: '/ request'
+      }
+    },
+
     // Channel Management
     channels: {
       title: 'Channel Management',