feat: add Claude Fable 5 model mapping across Claude Code and Desktop

- Wire claude-fable-5 as a fourth tier on both proxy paths, with a
  fable -> opus -> default fallback mirroring the official downgrade.
- Whitelist the fable- prefix for the Desktop 1.12603.1+ validator.
- Clarify fallbackModelHint (zh/en/ja/zh-TW): a blank tier on
  third-party endpoints forwards the literal model name and 404s.

Refs #3980, #4026, #4049.

src/config/claudeDesktopProviderPresets.ts

@@ -25,13 +25,16 @@ export interface ClaudeDesktopRoutePreset {
 }
 
 /**
- * Claude Desktop 3P fail-all 校验只接受 `claude-(sonnet|opus|haiku)-*` 形式的
- * routeId（1.6259.1+，实测 2026-05-13）。所有预设工厂、表单角色下拉、
- * 后端 `next_catalog_safe_route_id` 都从此映射派生 routeId，避免散落硬编码。
+ * Claude Desktop 3P fail-all 校验接受的角色名。Desktop 1.12603.1+ 起白名单
+ * 纳入 fable（app.asar 内 ["sonnet","opus","haiku","fable","mythos"]，实测
+ * 2026-06-13）；此前 1.6259.1 仅接受 sonnet/opus/haiku。mythos 官方未公开
+ * 发布，暂不暴露给用户。所有预设工厂、表单角色下拉、后端
+ * `next_catalog_safe_route_id` 都从此映射派生 routeId，避免散落硬编码。
  */
 export const CLAUDE_DESKTOP_ROLE_ROUTE_IDS = {
   sonnet: "claude-sonnet-4-6",
   opus: "claude-opus-4-8",
+  fable: "claude-fable-5",
   haiku: "claude-haiku-4-5",
 } as const;