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-TW.json

@@ -1892,6 +1892,7 @@
                 "viewOnHuggingFaceText": "在 Hugging Face 查看",
                 "viewCreatorProfile": "查看創作者個人檔案",
                 "openFileLocation": "開啟檔案位置",
+                "openSidecarLocation": "開啟中繼資料位置",
                 "sendToWorkflow": "傳送到 ComfyUI",
                 "sendToWorkflowText": "傳送到 ComfyUI",
                 "copyHash": "複製雜湊值",
@@ -1905,6 +1906,12 @@
                 "copied": "路徑已複製到剪貼簿：{{path}}",
                 "clipboardFallback": "路徑：{{path}}"
             },
+            "openSidecarLocation": {
+                "success": "中繼資料位置已成功開啟",
+                "failed": "開啟中繼資料位置失敗",
+                "copied": "路徑已複製到剪貼簿：{{path}}",
+                "clipboardFallback": "路徑：{{path}}"
+            },
             "sendToWorkflow": {
                 "noFilePath": "無法傳送到 ComfyUI：沒有可用的檔案路徑"
             },