feat(modal): open model metadata sidecar location from the model modal

Centralized sidecar storage moves .metadata.json into a mirror tree, so
the Location link in the model modal could no longer reveal a model's
sidecar. The Location row now shows an inline metadata icon button
(centralized mode only) that calls a new
POST /api/lm/models/open-sidecar-location endpoint: the backend resolves
the sidecar through py/utils/sidecar_paths, reveals the .metadata.json
selected in the file manager (falling back to the sidecar directory when
it has not been written yet), and returns the path for clipboard mode on
headless/Docker sessions. The platform reveal logic is factored into a
shared FileSystemHandler._reveal_file helper. All 9 locales translated.

locales/zh-CN.json

@@ -1892,6 +1892,7 @@
                 "viewOnHuggingFaceText": "在 Hugging Face 查看",
                 "viewCreatorProfile": "查看创作者主页",
                 "openFileLocation": "打开文件位置",
+                "openSidecarLocation": "打开元数据位置",
                 "sendToWorkflow": "发送到 ComfyUI",
                 "sendToWorkflowText": "发送到 ComfyUI",
                 "copyHash": "复制哈希值",
@@ -1905,6 +1906,12 @@
                 "copied": "路径已复制到剪贴板：{{path}}",
                 "clipboardFallback": "路径：{{path}}"
             },
+            "openSidecarLocation": {
+                "success": "元数据位置已成功打开",
+                "failed": "打开元数据位置失败",
+                "copied": "路径已复制到剪贴板：{{path}}",
+                "clipboardFallback": "路径：{{path}}"
+            },
             "sendToWorkflow": {
                 "noFilePath": "无法发送到 ComfyUI：没有可用的文件路径"
             },