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  filters out group IDs no longer active.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/admin/available_channel_handler_test.go

@@ -0,0 +1,57 @@
+//go:build unit
+
+package admin
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAvailableChannelToAdminResponse_IncludesFullDTO(t *testing.T) {
+	// 管理员视图应包含 id / status / billing_model_source / restrict_models 等
+	// 管理字段；BillingModelSource 为空时应默认回填 channel_mapped。
+	input := service.AvailableChannel{
+		ID:                 42,
+		Name:               "ch",
+		Description:        "d",
+		Status:             service.StatusActive,
+		BillingModelSource: "", // 验证默认值填充
+		RestrictModels:     true,
+		Groups: []service.AvailableGroupRef{
+			{ID: 1, Name: "g1", Platform: "anthropic"},
+		},
+		SupportedModels: []service.SupportedModel{
+			{Name: "claude-sonnet-4-6", Platform: "anthropic"},
+		},
+	}
+
+	resp := AvailableChannelToAdminResponse(input)
+	require.Equal(t, int64(42), resp.ID)
+	require.Equal(t, "ch", resp.Name)
+	require.Equal(t, service.StatusActive, resp.Status)
+	require.Equal(t, service.BillingModelSourceChannelMapped, resp.BillingModelSource)
+	require.True(t, resp.RestrictModels)
+	require.Len(t, resp.Groups, 1)
+	require.Len(t, resp.SupportedModels, 1)
+
+	// JSON 层验证管理字段确实会被序列化。
+	raw, err := json.Marshal(resp)
+	require.NoError(t, err)
+	var decoded map[string]any
+	require.NoError(t, json.Unmarshal(raw, &decoded))
+	for _, key := range []string{"id", "status", "billing_model_source", "restrict_models", "groups", "supported_models"} {
+		_, exists := decoded[key]
+		require.Truef(t, exists, "admin DTO must expose %q", key)
+	}
+}
+
+func TestAvailableChannelToAdminResponse_PreservesExplicitBillingSource(t *testing.T) {
+	input := service.AvailableChannel{
+		BillingModelSource: service.BillingModelSourceUpstream,
+	}
+	resp := AvailableChannelToAdminResponse(input)
+	require.Equal(t, service.BillingModelSourceUpstream, resp.BillingModelSource)
+}