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-tauri/src/proxy/model_mapper.rs

@@ -11,6 +11,7 @@ pub struct ModelMapping {
     pub haiku_model: Option<String>,
     pub sonnet_model: Option<String>,
     pub opus_model: Option<String>,
+    pub fable_model: Option<String>,
     pub default_model: Option<String>,
 }
 
@@ -35,6 +36,11 @@ impl ModelMapping {
                 .and_then(|v| v.as_str())
                 .filter(|s| !s.is_empty())
                 .map(String::from),
+            fable_model: env
+                .and_then(|e| e.get("ANTHROPIC_DEFAULT_FABLE_MODEL"))
+                .and_then(|v| v.as_str())
+                .filter(|s| !s.is_empty())
+                .map(String::from),
             default_model: env
                 .and_then(|e| e.get("ANTHROPIC_MODEL"))
                 .and_then(|v| v.as_str())
@@ -48,6 +54,7 @@ impl ModelMapping {
         self.haiku_model.is_some()
             || self.sonnet_model.is_some()
             || self.opus_model.is_some()
+            || self.fable_model.is_some()
             || self.default_model.is_some()
     }
 
@@ -56,6 +63,16 @@ impl ModelMapping {
         let model_lower = original_model.to_lowercase();
 
         // 1. 按模型类型匹配
+        if model_lower.contains("fable") {
+            if let Some(ref m) = self.fable_model {
+                return m.clone();
+            }
+            // 未单独配置 fable 档时归入 opus 档，与 Claude Code 官方
+            // 分类器降级方向一致（fable→opus），避免落到 default 失去层级。
+            if let Some(ref m) = self.opus_model {
+                return m.clone();
+            }
+        }
         if model_lower.contains("haiku") {
             if let Some(ref m) = self.haiku_model {
                 return m.clone();
@@ -154,7 +171,8 @@ mod tests {
                     "ANTHROPIC_MODEL": "default-model",
                     "ANTHROPIC_DEFAULT_HAIKU_MODEL": "haiku-mapped",
                     "ANTHROPIC_DEFAULT_SONNET_MODEL": "sonnet-mapped",
-                    "ANTHROPIC_DEFAULT_OPUS_MODEL": "opus-mapped"
+                    "ANTHROPIC_DEFAULT_OPUS_MODEL": "opus-mapped",
+                    "ANTHROPIC_DEFAULT_FABLE_MODEL": "fable-mapped"
                 }
             }),
             website_url: None,
@@ -214,6 +232,54 @@ mod tests {
         assert_eq!(mapped, Some("opus-mapped".to_string()));
     }
 
+    #[test]
+    fn test_fable_mapping() {
+        let provider = create_provider_with_mapping();
+        let body = json!({"model": "claude-fable-5"});
+        let (result, _, mapped) = apply_model_mapping(body, &provider);
+        assert_eq!(result["model"], "fable-mapped");
+        assert_eq!(mapped, Some("fable-mapped".to_string()));
+    }
+
+    #[test]
+    fn test_fable_with_one_m_suffix_mapping() {
+        // Claude Code 实际会发 claude-fable-5[1m] 形态（issue #3980）
+        let provider = create_provider_with_mapping();
+        let body = json!({"model": "claude-fable-5[1m]"});
+        let (result, _, mapped) = apply_model_mapping(body, &provider);
+        assert_eq!(result["model"], "fable-mapped");
+        assert_eq!(mapped, Some("fable-mapped".to_string()));
+    }
+
+    #[test]
+    fn test_fable_falls_back_to_opus_when_unset() {
+        let mut provider = create_provider_with_mapping();
+        provider.settings_config = json!({
+            "env": {
+                "ANTHROPIC_MODEL": "default-model",
+                "ANTHROPIC_DEFAULT_OPUS_MODEL": "opus-mapped"
+            }
+        });
+        let body = json!({"model": "claude-fable-5"});
+        let (result, _, mapped) = apply_model_mapping(body, &provider);
+        assert_eq!(result["model"], "opus-mapped");
+        assert_eq!(mapped, Some("opus-mapped".to_string()));
+    }
+
+    #[test]
+    fn test_fable_falls_back_to_default_without_opus() {
+        let mut provider = create_provider_with_mapping();
+        provider.settings_config = json!({
+            "env": {
+                "ANTHROPIC_MODEL": "default-model"
+            }
+        });
+        let body = json!({"model": "claude-fable-5"});
+        let (result, _, mapped) = apply_model_mapping(body, &provider);
+        assert_eq!(result["model"], "default-model");
+        assert_eq!(mapped, Some("default-model".to_string()));
+    }
+
     #[test]
     fn test_thinking_does_not_affect_model_mapping() {
         // Issue #2081: thinking 参数不应影响模型映射