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/components/providers/forms/hooks/useModelState.ts

@@ -12,7 +12,9 @@ export type ClaudeModelEnvField =
   | "ANTHROPIC_DEFAULT_SONNET_MODEL"
   | "ANTHROPIC_DEFAULT_SONNET_MODEL_NAME"
   | "ANTHROPIC_DEFAULT_OPUS_MODEL"
-  | "ANTHROPIC_DEFAULT_OPUS_MODEL_NAME";
+  | "ANTHROPIC_DEFAULT_OPUS_MODEL_NAME"
+  | "ANTHROPIC_DEFAULT_FABLE_MODEL"
+  | "ANTHROPIC_DEFAULT_FABLE_MODEL_NAME";
 
 export const CLAUDE_ONE_M_MARKER = "[1M]";
 
@@ -69,8 +71,28 @@ function parseModelsFromConfig(settingsConfig: string) {
       typeof env.ANTHROPIC_DEFAULT_OPUS_MODEL_NAME === "string"
         ? env.ANTHROPIC_DEFAULT_OPUS_MODEL_NAME
         : stripClaudeOneMMarker(opus);
+    // 回填链镜像运行时映射链（fable → opus → default），保证 UI 展示
+    // 与代理实际转发的模型一致。
+    const fable =
+      typeof env.ANTHROPIC_DEFAULT_FABLE_MODEL === "string"
+        ? env.ANTHROPIC_DEFAULT_FABLE_MODEL
+        : opus;
+    const fableName =
+      typeof env.ANTHROPIC_DEFAULT_FABLE_MODEL_NAME === "string"
+        ? env.ANTHROPIC_DEFAULT_FABLE_MODEL_NAME
+        : stripClaudeOneMMarker(fable);
 
-    return { model, haiku, haikuName, sonnet, sonnetName, opus, opusName };
+    return {
+      model,
+      haiku,
+      haikuName,
+      sonnet,
+      sonnetName,
+      opus,
+      opusName,
+      fable,
+      fableName,
+    };
   } catch {
     return {
       model: "",
@@ -80,6 +102,8 @@ function parseModelsFromConfig(settingsConfig: string) {
       sonnetName: "",
       opus: "",
       opusName: "",
+      fable: "",
+      fableName: "",
     };
   }
 }
@@ -106,6 +130,10 @@ export function useModelState({
   const [defaultOpusModelName, setDefaultOpusModelName] = useState(
     initial.opusName,
   );
+  const [defaultFableModel, setDefaultFableModel] = useState(initial.fable);
+  const [defaultFableModelName, setDefaultFableModelName] = useState(
+    initial.fableName,
+  );
 
   const isUserEditingRef = useRef(false);
   const lastConfigRef = useRef(settingsConfig);
@@ -134,6 +162,8 @@ export function useModelState({
     setDefaultSonnetModelName(parsed.sonnetName);
     setDefaultOpusModel(parsed.opus);
     setDefaultOpusModelName(parsed.opusName);
+    setDefaultFableModel(parsed.fable);
+    setDefaultFableModelName(parsed.fableName);
   }, [settingsConfig]);
 
   const handleModelChange = useCallback(
@@ -152,6 +182,10 @@ export function useModelState({
       if (field === "ANTHROPIC_DEFAULT_OPUS_MODEL") setDefaultOpusModel(value);
       if (field === "ANTHROPIC_DEFAULT_OPUS_MODEL_NAME")
         setDefaultOpusModelName(value);
+      if (field === "ANTHROPIC_DEFAULT_FABLE_MODEL")
+        setDefaultFableModel(value);
+      if (field === "ANTHROPIC_DEFAULT_FABLE_MODEL_NAME")
+        setDefaultFableModelName(value);
 
       try {
         const currentConfig = latestConfigRef.current
@@ -195,6 +229,10 @@ export function useModelState({
     setDefaultOpusModel,
     defaultOpusModelName,
     setDefaultOpusModelName,
+    defaultFableModel,
+    setDefaultFableModel,
+    defaultFableModelName,
+    setDefaultFableModelName,
     handleModelChange,
   };
 }