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  filters out group IDs no longer active.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/handler/wire.go

@@ -36,6 +36,7 @@ func ProvideAdminHandlers(
 	channelHandler *admin.ChannelHandler,
 	channelMonitorHandler *admin.ChannelMonitorHandler,
 	channelMonitorTemplateHandler *admin.ChannelMonitorRequestTemplateHandler,
+	availableChannelHandler *admin.AvailableChannelHandler,
 	paymentHandler *admin.PaymentHandler,
 ) *AdminHandlers {
 	return &AdminHandlers{
@@ -66,6 +67,7 @@ func ProvideAdminHandlers(
 		Channel:                channelHandler,
 		ChannelMonitor:         channelMonitorHandler,
 		ChannelMonitorTemplate: channelMonitorTemplateHandler,
+		AvailableChannel:       availableChannelHandler,
 		Payment:                paymentHandler,
 	}
 }
@@ -97,25 +99,27 @@ func ProvideHandlers(
 	totpHandler *TotpHandler,
 	paymentHandler *PaymentHandler,
 	paymentWebhookHandler *PaymentWebhookHandler,
+	availableChannelHandler *AvailableChannelHandler,
 	_ *service.IdempotencyCoordinator,
 	_ *service.IdempotencyCleanupService,
 ) *Handlers {
 	return &Handlers{
-		Auth:           authHandler,
-		User:           userHandler,
-		APIKey:         apiKeyHandler,
-		Usage:          usageHandler,
-		Redeem:         redeemHandler,
-		Subscription:   subscriptionHandler,
-		Announcement:   announcementHandler,
-		ChannelMonitor: channelMonitorUserHandler,
-		Admin:          adminHandlers,
-		Gateway:        gatewayHandler,
-		OpenAIGateway:  openaiGatewayHandler,
-		Setting:        settingHandler,
-		Totp:           totpHandler,
-		Payment:        paymentHandler,
-		PaymentWebhook: paymentWebhookHandler,
+		Auth:             authHandler,
+		User:             userHandler,
+		APIKey:           apiKeyHandler,
+		Usage:            usageHandler,
+		Redeem:           redeemHandler,
+		Subscription:     subscriptionHandler,
+		Announcement:     announcementHandler,
+		ChannelMonitor:   channelMonitorUserHandler,
+		Admin:            adminHandlers,
+		Gateway:          gatewayHandler,
+		OpenAIGateway:    openaiGatewayHandler,
+		Setting:          settingHandler,
+		Totp:             totpHandler,
+		Payment:          paymentHandler,
+		PaymentWebhook:   paymentWebhookHandler,
+		AvailableChannel: availableChannelHandler,
 	}
 }
 
@@ -136,6 +140,7 @@ var ProviderSet = wire.NewSet(
 	ProvideSettingHandler,
 	NewPaymentHandler,
 	NewPaymentWebhookHandler,
+	NewAvailableChannelHandler,
 
 	// Admin handlers
 	admin.NewDashboardHandler,
@@ -165,6 +170,7 @@ var ProviderSet = wire.NewSet(
 	admin.NewChannelHandler,
 	admin.NewChannelMonitorHandler,
 	admin.NewChannelMonitorRequestTemplateHandler,
+	admin.NewAvailableChannelHandler,
 	admin.NewPaymentHandler,
 
 	// AdminHandlers and Handlers constructors