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/ProviderForm.tsx

@@ -449,6 +449,8 @@ function ProviderFormFull({
     defaultSonnetModelName,
     defaultOpusModel,
     defaultOpusModelName,
+    defaultFableModel,
+    defaultFableModelName,
     handleModelChange,
   } = useModelState({
     settingsConfig: form.getValues("settingsConfig"),
@@ -2009,6 +2011,8 @@ function ProviderFormFull({
               defaultSonnetModelName={defaultSonnetModelName}
               defaultOpusModel={defaultOpusModel}
               defaultOpusModelName={defaultOpusModelName}
+              defaultFableModel={defaultFableModel}
+              defaultFableModelName={defaultFableModelName}
               onModelChange={handleModelChange}
               speedTestEndpoints={speedTestEndpoints}
               apiFormat={localApiFormat}