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  filters out group IDs no longer active.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_available.go

@@ -0,0 +1,84 @@
+package service
+
+import (
+	"context"
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// AvailableGroupRef 渠道视图中关联分组的简要信息。
+type AvailableGroupRef struct {
+	ID       int64
+	Name     string
+	Platform string
+}
+
+// AvailableChannel 可用渠道视图：用于「可用渠道」页面展示渠道基础信息 +
+// 关联的分组 + 推导出的支持模型列表（无通配符）。
+type AvailableChannel struct {
+	ID                 int64
+	Name               string
+	Description        string
+	Status             string
+	BillingModelSource string
+	RestrictModels     bool
+	Groups             []AvailableGroupRef
+	SupportedModels    []SupportedModel
+}
+
+// ListAvailable 返回所有渠道的可用视图：每个渠道附带关联分组信息与支持模型列表。
+//
+// 支持模型通过 (*Channel).SupportedModels() 计算得到（见 channel.go）。
+// 关联分组信息通过 groupRepo.ListActive 查询后按 ID 映射；渠道 GroupIDs 中未在活跃列表中
+// 的分组（已停用或删除）会被忽略。
+func (s *ChannelService) ListAvailable(ctx context.Context) ([]AvailableChannel, error) {
+	channels, err := s.repo.ListAll(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("list channels: %w", err)
+	}
+
+	groupByID := make(map[int64]AvailableGroupRef)
+	if s.groupRepo != nil {
+		groups, err := s.groupRepo.ListActive(ctx)
+		if err != nil {
+			return nil, fmt.Errorf("list active groups: %w", err)
+		}
+		for i := range groups {
+			g := groups[i]
+			groupByID[g.ID] = AvailableGroupRef{
+				ID:       g.ID,
+				Name:     g.Name,
+				Platform: g.Platform,
+			}
+		}
+	}
+
+	out := make([]AvailableChannel, 0, len(channels))
+	for i := range channels {
+		ch := &channels[i]
+		groups := make([]AvailableGroupRef, 0, len(ch.GroupIDs))
+		for _, gid := range ch.GroupIDs {
+			if ref, ok := groupByID[gid]; ok {
+				groups = append(groups, ref)
+			}
+		}
+		sort.Slice(groups, func(i, j int) bool { return groups[i].Name < groups[j].Name })
+
+		out = append(out, AvailableChannel{
+			ID:                 ch.ID,
+			Name:               ch.Name,
+			Description:        ch.Description,
+			Status:             ch.Status,
+			BillingModelSource: ch.BillingModelSource,
+			RestrictModels:     ch.RestrictModels,
+			Groups:             groups,
+			SupportedModels:    ch.SupportedModels(),
+		})
+	}
+
+	sort.SliceStable(out, func(i, j int) bool {
+		return strings.ToLower(out[i].Name) < strings.ToLower(out[j].Name)
+	})
+	return out, nil
+}