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/ja.json

@@ -1892,6 +1892,7 @@
                 "viewOnHuggingFaceText": "Hugging Face で見る",
                 "viewCreatorProfile": "作成者プロフィールを表示",
                 "openFileLocation": "ファイルの場所を開く",
+                "openSidecarLocation": "メタデータの場所を開く",
                 "sendToWorkflow": "ComfyUI に送信",
                 "sendToWorkflowText": "ComfyUI に送信",
                 "copyHash": "ハッシュをコピー",
@@ -1905,6 +1906,12 @@
                 "copied": "パスをクリップボードにコピーしました: {{path}}",
                 "clipboardFallback": "パス: {{path}}"
             },
+            "openSidecarLocation": {
+                "success": "メタデータの場所を正常に開きました",
+                "failed": "メタデータの場所を開くのに失敗しました",
+                "copied": "パスをクリップボードにコピーしました: {{path}}",
+                "clipboardFallback": "パス: {{path}}"
+            },
             "sendToWorkflow": {
                 "noFilePath": "ComfyUI に送信できません：ファイルパスがありません"
             },