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  filters out group IDs no longer active.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

backend/internal/service/channel_service.go

@@ -141,6 +141,7 @@ const (
 // ChannelService 渠道管理服务
 type ChannelService struct {
 	repo                 ChannelRepository
+	groupRepo            GroupRepository
 	authCacheInvalidator APIKeyAuthCacheInvalidator
 
 	cache   atomic.Value // *channelCache
@@ -148,9 +149,10 @@ type ChannelService struct {
 }
 
 // NewChannelService 创建渠道服务实例
-func NewChannelService(repo ChannelRepository, authCacheInvalidator APIKeyAuthCacheInvalidator) *ChannelService {
+func NewChannelService(repo ChannelRepository, groupRepo GroupRepository, authCacheInvalidator APIKeyAuthCacheInvalidator) *ChannelService {
 	s := &ChannelService{
 		repo:                 repo,
+		groupRepo:            groupRepo,
 		authCacheInvalidator: authCacheInvalidator,
 	}
 	return s
@@ -884,12 +886,7 @@ func conflictsBetween(a, b modelEntry) bool {
 
 // toModelEntry 将模型名转换为 modelEntry
 func toModelEntry(pattern string) modelEntry {
-	lower := strings.ToLower(pattern)
-	isWild := strings.HasSuffix(lower, "*")
-	prefix := lower
-	if isWild {
-		prefix = strings.TrimSuffix(lower, "*")
-	}
+	prefix, isWild := splitWildcardSuffix(strings.ToLower(pattern))
 	return modelEntry{pattern: pattern, prefix: prefix, wildcard: isWild}
 }