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/en.json

@@ -1892,6 +1892,7 @@
         "viewOnHuggingFaceText": "View on Hugging Face",
         "viewCreatorProfile": "View Creator Profile",
         "openFileLocation": "Open File Location",
+        "openSidecarLocation": "Open metadata location",
         "sendToWorkflow": "Send to ComfyUI",
         "sendToWorkflowText": "Send to ComfyUI",
         "copyHash": "Copy hash",
@@ -1905,6 +1906,12 @@
         "copied": "Path copied to clipboard: {{path}}",
         "clipboardFallback": "Path: {{path}}"
       },
+      "openSidecarLocation": {
+        "success": "Metadata location opened successfully",
+        "failed": "Failed to open metadata location",
+        "copied": "Path copied to clipboard: {{path}}",
+        "clipboardFallback": "Path: {{path}}"
+      },
       "sendToWorkflow": {
         "noFilePath": "Unable to send to ComfyUI: No file path available"
       },