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/de.json

@@ -1892,6 +1892,7 @@
                 "viewOnHuggingFaceText": "Auf Hugging Face ansehen",
                 "viewCreatorProfile": "Ersteller-Profil anzeigen",
                 "openFileLocation": "Dateispeicherort öffnen",
+                "openSidecarLocation": "Metadaten-Speicherort öffnen",
                 "sendToWorkflow": "An ComfyUI senden",
                 "sendToWorkflowText": "An ComfyUI senden",
                 "copyHash": "Hash kopieren",
@@ -1905,6 +1906,12 @@
                 "copied": "Pfad in die Zwischenablage kopiert: {{path}}",
                 "clipboardFallback": "Pfad: {{path}}"
             },
+            "openSidecarLocation": {
+                "success": "Metadaten-Speicherort erfolgreich geöffnet",
+                "failed": "Fehler beim Öffnen des Metadaten-Speicherorts",
+                "copied": "Pfad in die Zwischenablage kopiert: {{path}}",
+                "clipboardFallback": "Pfad: {{path}}"
+            },
             "sendToWorkflow": {
                 "noFilePath": "Kann nicht an ComfyUI senden: Kein Dateipfad verfügbar"
             },