feat: 支持皮肤包就地更新与新建即传

- 新增皮肤包更新接口，覆盖内容保持文件名不变，避免 APP 配置的主题引用失效
- 上传新皮肤后自动为空主题目录的证书选中该皮肤
- 新建 APP 保存后不关弹窗并回填 upload_key，可直接上传皮肤

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/routers/config.py

@@ -316,7 +316,7 @@ async def create_app(app: dict):
         apps[new_id] = app
         config["apps"] = apps
         save_config(config)
-    return {"id": new_id, "message": "App 已创建"}
+    return {"id": new_id, "upload_key": app["upload_key"], "message": "App 已创建"}
 
 
 @router.put("/apps/{app_id}")
@@ -410,6 +410,33 @@ async def upload_skin(upload_key: str, file: UploadFile = File(...)):
     return {"message": "皮肤包上传成功", "name": file.filename}
 
 
+@router.put("/skins/{upload_key}/{skin_name}")
+async def replace_skin(upload_key: str, skin_name: str, file: UploadFile = File(...)):
+    """替换已有皮肤包内容，保持原文件名不变。
+
+    APP 配置的 theme 通过文件名（去掉 .zip）引用皮肤包，若上传新文件名会新增一个皮肤而非更新，
+    导致原引用失效。此接口就地覆盖指定皮肤，确保被引用的皮肤可以更新。
+    """
+    if not file.filename or not file.filename.lower().endswith(".zip"):
+        raise HTTPException(status_code=400, detail="仅支持 .zip 格式")
+
+    config = load_config()
+    app_id, _ = _find_app_by_upload_key(config, upload_key)
+    if not app_id:
+        raise HTTPException(status_code=404, detail="App 不存在")
+
+    app_skins_dir = SKINS_DIR / upload_key
+    skin_path = app_skins_dir / skin_name
+    if not skin_path.exists():
+        raise HTTPException(status_code=404, detail="皮肤包不存在")
+
+    content = await file.read()
+    with open(skin_path, "wb") as f:
+        f.write(content)
+
+    return {"message": "皮肤包已更新", "name": skin_name}
+
+
 @router.delete("/skins/{upload_key}/{skin_name}")
 async def delete_skin(upload_key: str, skin_name: str):
     """删除指定皮肤包"""