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  filters out group IDs no longer active.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/cmd/server/wire_gen.go

@@ -174,7 +174,8 @@ func initializeApplication(buildInfo handler.BuildInfo) (*Application, error) {
 	claudeTokenProvider := service.ProvideClaudeTokenProvider(accountRepository, geminiTokenCache, oAuthService, oAuthRefreshAPI)
 	digestSessionStore := service.NewDigestSessionStore()
 	channelRepository := repository.NewChannelRepository(db)
-	channelService := service.NewChannelService(channelRepository, apiKeyAuthCacheInvalidator)
+	channelService := service.NewChannelService(channelRepository, groupRepository, apiKeyAuthCacheInvalidator)
+	availableChannelHandler := admin.NewAvailableChannelHandler(channelService)
 	modelPricingResolver := service.NewModelPricingResolver(channelService, billingService)
 	balanceNotifyService := service.ProvideBalanceNotifyService(emailService, settingRepository, accountRepository)
 	gatewayService := service.NewGatewayService(accountRepository, groupRepository, usageLogRepository, usageBillingRepository, userRepository, userSubscriptionRepository, userGroupRateRepository, gatewayCache, configConfig, schedulerSnapshotService, concurrencyService, billingService, rateLimitService, billingCacheService, identityService, httpUpstream, deferredService, claudeTokenProvider, sessionLimitCache, rpmCache, digestSessionStore, settingService, tlsFingerprintProfileService, channelService, modelPricingResolver, balanceNotifyService)
@@ -234,7 +235,8 @@ func initializeApplication(buildInfo handler.BuildInfo) (*Application, error) {
 	settingHandler := admin.NewSettingHandler(settingService, emailService, turnstileService, opsService, paymentConfigService, paymentService)
 	paymentOrderExpiryService := service.ProvidePaymentOrderExpiryService(paymentService)
 	paymentHandler := admin.NewPaymentHandler(paymentService, paymentConfigService)
-	adminHandlers := handler.ProvideAdminHandlers(dashboardHandler, adminUserHandler, groupHandler, accountHandler, adminAnnouncementHandler, dataManagementHandler, backupHandler, oAuthHandler, openAIOAuthHandler, geminiOAuthHandler, antigravityOAuthHandler, proxyHandler, adminRedeemHandler, promoHandler, settingHandler, opsHandler, systemHandler, adminSubscriptionHandler, adminUsageHandler, userAttributeHandler, errorPassthroughHandler, tlsFingerprintProfileHandler, adminAPIKeyHandler, scheduledTestHandler, channelHandler, channelMonitorHandler, channelMonitorRequestTemplateHandler, paymentHandler)
+	availableChannelUserHandler := handler.NewAvailableChannelHandler(channelService, apiKeyService)
+	adminHandlers := handler.ProvideAdminHandlers(dashboardHandler, adminUserHandler, groupHandler, accountHandler, adminAnnouncementHandler, dataManagementHandler, backupHandler, oAuthHandler, openAIOAuthHandler, geminiOAuthHandler, antigravityOAuthHandler, proxyHandler, adminRedeemHandler, promoHandler, settingHandler, opsHandler, systemHandler, adminSubscriptionHandler, adminUsageHandler, userAttributeHandler, errorPassthroughHandler, tlsFingerprintProfileHandler, adminAPIKeyHandler, scheduledTestHandler, channelHandler, channelMonitorHandler, channelMonitorRequestTemplateHandler, availableChannelHandler, paymentHandler)
 	usageRecordWorkerPool := service.NewUsageRecordWorkerPool(configConfig)
 	userMsgQueueCache := repository.NewUserMsgQueueCache(redisClient)
 	userMessageQueueService := service.ProvideUserMessageQueueService(userMsgQueueCache, rpmCache, configConfig)
@@ -246,7 +248,7 @@ func initializeApplication(buildInfo handler.BuildInfo) (*Application, error) {
 	paymentWebhookHandler := handler.NewPaymentWebhookHandler(paymentService, registry)
 	idempotencyCoordinator := service.ProvideIdempotencyCoordinator(idempotencyRepository, configConfig)
 	idempotencyCleanupService := service.ProvideIdempotencyCleanupService(idempotencyRepository, configConfig)
-	handlers := handler.ProvideHandlers(authHandler, userHandler, apiKeyHandler, usageHandler, redeemHandler, subscriptionHandler, announcementHandler, channelMonitorUserHandler, adminHandlers, gatewayHandler, openAIGatewayHandler, handlerSettingHandler, totpHandler, handlerPaymentHandler, paymentWebhookHandler, idempotencyCoordinator, idempotencyCleanupService)
+	handlers := handler.ProvideHandlers(authHandler, userHandler, apiKeyHandler, usageHandler, redeemHandler, subscriptionHandler, announcementHandler, channelMonitorUserHandler, adminHandlers, gatewayHandler, openAIGatewayHandler, handlerSettingHandler, totpHandler, handlerPaymentHandler, paymentWebhookHandler, availableChannelUserHandler, idempotencyCoordinator, idempotencyCleanupService)
 	jwtAuthMiddleware := middleware.NewJWTAuthMiddleware(authService, userService)
 	adminAuthMiddleware := middleware.NewAdminAuthMiddleware(authService, userService, settingService)
 	apiKeyAuthMiddleware := middleware.NewAPIKeyAuthMiddleware(apiKeyService, subscriptionService, configConfig)