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.

tests/components/ClaudeDesktopProviderForm.test.tsx

@@ -97,7 +97,7 @@ describe("ClaudeDesktopProviderForm", () => {
     expect(document.activeElement).toBe(currentInput);
   });
 
-  it("代理模式始终渲染 Sonnet / Opus / Haiku 三档（即使只配了一档）", () => {
+  it("代理模式始终渲染 Sonnet / Opus / Fable / Haiku 四档（即使只配了一档）", () => {
     renderForm({
       name: "Proxy Provider",
       settingsConfig: {
@@ -114,13 +114,13 @@ describe("ClaudeDesktopProviderForm", () => {
       },
     });
 
-    // 固定三档：每档各一个「菜单显示名」输入框，无论初始只配了几档。
-    expect(screen.getAllByPlaceholderText("DeepSeek V4 Pro")).toHaveLength(3);
+    // 固定四档：每档各一个「菜单显示名」输入框，无论初始只配了几档。
+    expect(screen.getAllByPlaceholderText("DeepSeek V4 Pro")).toHaveLength(4);
   });
 
-  it("代理模式初始无路由且默认路由未就绪时不渲染空三档", () => {
+  it("代理模式初始无路由且默认路由未就绪时不渲染空四档", () => {
     // mock 的 getClaudeDesktopDefaultRoutes 返回 []，模拟默认路由尚未就绪。
-    // 修复前：normalizeProxyRows([]) 会渲染 3 条空行并把 routes.length 撑到 3，
+    // 修复前：normalizeProxyRows([]) 会渲染空行并把 routes.length 撑起来，
     // 永久挡住 seed effect 的默认路由回填。修复后应保持空、等待 seed。
     renderForm({
       name: "Proxy Provider",
@@ -139,7 +139,7 @@ describe("ClaudeDesktopProviderForm", () => {
     expect(screen.queryAllByPlaceholderText("DeepSeek V4 Pro")).toHaveLength(0);
   });
 
-  it("保存模型映射时补齐固定三档并把留空档回填为 Sonnet 模型", async () => {
+  it("保存模型映射时补齐固定四档并把留空档回填为 Sonnet 模型", async () => {
     const onSubmit = vi.fn();
     renderForm(
       {
@@ -166,19 +166,25 @@ describe("ClaudeDesktopProviderForm", () => {
 
     await waitFor(() => expect(onSubmit).toHaveBeenCalled());
     const submitted = onSubmit.mock.calls[0][0];
-    // claude-old 迁移到 Sonnet；留空的 Opus / Haiku 回填为 Sonnet 的上游模型，
-    // 保证落库三档齐全，子 agent 调用的 Haiku 始终可解析。
+    // claude-old 迁移到 Sonnet；留空的 Opus / Fable / Haiku 回填为 Sonnet 的
+    // 上游模型，保证落库四档齐全，子 agent 调用的各档始终可解析。
     expect(submitted.meta.claudeDesktopModelRoutes).toMatchObject({
       "claude-sonnet-4-6": {
         model: "upstream-old",
         labelOverride: "upstream-old",
       },
       "claude-opus-4-8": { model: "upstream-old" },
+      "claude-fable-5": { model: "upstream-old" },
       "claude-haiku-4-5": { model: "upstream-old" },
     });
     expect(
       Object.keys(submitted.meta.claudeDesktopModelRoutes).sort(),
-    ).toEqual(["claude-haiku-4-5", "claude-opus-4-8", "claude-sonnet-4-6"]);
+    ).toEqual([
+      "claude-fable-5",
+      "claude-haiku-4-5",
+      "claude-opus-4-8",
+      "claude-sonnet-4-6",
+    ]);
   });
 
   it("回填空档时继承 Sonnet 的 1M 声明", async () => {

tests/components/ClaudeFormFields.test.tsx

@@ -83,6 +83,8 @@ const renderCopilotForm = (overrides: Partial<ClaudeFormFieldsProps> = {}) => {
     defaultSonnetModelName: "Claude Sonnet",
     defaultOpusModel: "",
     defaultOpusModelName: "",
+    defaultFableModel: "",
+    defaultFableModelName: "",
     onModelChange: vi.fn(),
     speedTestEndpoints: [],
     apiFormat: "anthropic",