fix: allow model switching without re-entering wallet key

Users with existing wallets could not switch AI models because the
"Start Trading" button required a valid private key even when one was
already configured. Now the button is enabled when hasExistingWallet
is true, and handleSubmit passes an empty key so the backend preserves
the existing key.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/src/components/trader/ModelConfigModal.tsx

@@ -77,8 +77,11 @@ export function ModelConfigModal({
 
   const handleSubmit = (e: React.FormEvent) => {
     e.preventDefault()
-    if (!selectedModelId || !apiKey.trim()) return
-    onSave(selectedModelId, apiKey.trim(), baseUrl.trim() || undefined, modelName.trim() || undefined)
+    if (!selectedModelId) return
+    const key = apiKey.trim()
+    // Allow empty key when editing an existing model (backend preserves existing key)
+    if (!key && !editingModelId) return
+    onSave(selectedModelId, key, baseUrl.trim() || undefined, modelName.trim() || undefined)
   }
 
   const availableModels = allModels || []
@@ -833,9 +836,9 @@ function Claw402ConfigForm({
         </button>
         <button
           type="submit"
-          disabled={!isKeyValid}
+          disabled={!isKeyValid && !hasExistingWallet}
           className="flex-1 flex items-center justify-center gap-2 px-4 py-3 rounded-xl text-sm font-bold transition-all hover:scale-[1.02] disabled:opacity-50 disabled:cursor-not-allowed"
-          style={{ background: isKeyValid ? 'linear-gradient(135deg, #2563EB, #7C3AED)' : '#2B3139', color: '#fff' }}
+          style={{ background: (isKeyValid || hasExistingWallet) ? 'linear-gradient(135deg, #2563EB, #7C3AED)' : '#2B3139', color: '#fff' }}
         >
           {'🚀 ' + t('modelConfig.startTrading', language)}
         </button>