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/ko.json

@@ -1892,6 +1892,7 @@
                 "viewOnHuggingFaceText": "Hugging Face에서 보기",
                 "viewCreatorProfile": "제작자 프로필 보기",
                 "openFileLocation": "파일 위치 열기",
+                "openSidecarLocation": "메타데이터 위치 열기",
                 "sendToWorkflow": "ComfyUI로 보내기",
                 "sendToWorkflowText": "ComfyUI로 보내기",
                 "copyHash": "해시 복사",
@@ -1905,6 +1906,12 @@
                 "copied": "경로가 클립보드에 복사되었습니다: {{path}}",
                 "clipboardFallback": "경로: {{path}}"
             },
+            "openSidecarLocation": {
+                "success": "메타데이터 위치가 성공적으로 열렸습니다",
+                "failed": "메타데이터 위치 열기에 실패했습니다",
+                "copied": "경로가 클립보드에 복사되었습니다: {{path}}",
+                "clipboardFallback": "경로: {{path}}"
+            },
             "sendToWorkflow": {
                 "noFilePath": "ComfyUI로 보낼 수 없습니다: 파일 경로가 없습니다"
             },