feat(settings): directory picker and live validation for path settings

Add a reusable directory-picker modal backed by a new generic
POST /api/lm/browse-directory endpoint (browse logic extracted from the
recipe batch-import handler into py/utils/directory_browser.py) and wire
a browse button plus advisory validate-path feedback (POST
/api/lm/validate-path) into the settings path inputs: recipes path,
example images path/local root, and the extra-folder/model-path rows.

The browse button insets into the right edge of static inputs so narrow
settings rows keep their single-control layout.

Translations for the new settings.directoryPicker and
settings.pathValidation keys are filled in for all 9 locales.

locales/es.json

@@ -608,6 +608,22 @@
                 "controlnet": "Rutas de ControlNet"
             }
         },
+        "directoryPicker": {
+            "title": "Explorar carpetas",
+            "selectFolder": "Seleccionar esta carpeta",
+            "goUp": "Subir",
+            "pathPlaceholder": "Introducir ruta...",
+            "go": "Ir",
+            "emptyFolder": "No hay subcarpetas",
+            "loadError": "Error al cargar el directorio"
+        },
+        "pathValidation": {
+            "valid": "La ruta es válida",
+            "pathNotFound": "La ruta no existe",
+            "notADirectory": "No es un directorio",
+            "notReadable": "La ruta no es legible",
+            "notWritable": "La ruta no es escribible"
+        },
         "priorityTags": {
             "title": "Etiquetas prioritarias",
             "description": "Personaliza el orden de prioridad de etiquetas para cada tipo de modelo (p. ej., character, concept, style(toon|toon_style))",