feat(channels): explode available channels by platform + apply platform theme

Backend: one source channel → N output rows, one per platform that
has user-visible groups. Each row carries a single platform, so the
frontend can color/icon an entire row without mixing sources.

- userAvailableChannel: add Platform field
- new explodeChannelByPlatform helper; drop now-redundant
  collectGroupPlatforms

Frontend: use the row platform to drive theming and stop repeating
"ANTHROPIC" / "OPENAI" labels on every model chip.

- api/channels.ts: UserAvailableChannel.platform
- AvailableChannelsTable: name cell — PlatformBadge next to channel
  name (replaces the two-line name/description block; description
  moves to the badge's title tooltip); groups cell — each chip uses
  platformBadgeLightClass + PlatformIcon; model list passes
  show-platform=false + platform-hint to child chips
- SupportedModelChip: chip bg/border driven by platformBadgeClass,
  leading PlatformIcon; platform-hint fallback when model.platform
  missing

frontend/src/api/channels.ts

@@ -43,6 +43,11 @@ export interface UserSupportedModel {
 export interface UserAvailableChannel {
   name: string
   description: string
+  /**
+   * 所属平台（anthropic / openai / antigravity / gemini ...）。后端按平台把一个渠道
+   * 摊开成多条记录，因此此字段决定整行的配色与图标。
+   */
+  platform: string
   groups: UserAvailableGroup[]
   supported_models: UserSupportedModel[]
 }