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/ru.json

@@ -608,6 +608,22 @@
                 "controlnet": "Пути ControlNet"
             }
         },
+        "directoryPicker": {
+            "title": "Обзор папок",
+            "selectFolder": "Выбрать эту папку",
+            "goUp": "Вверх",
+            "pathPlaceholder": "Введите путь...",
+            "go": "Перейти",
+            "emptyFolder": "Нет подпапок",
+            "loadError": "Не удалось загрузить каталог"
+        },
+        "pathValidation": {
+            "valid": "Путь действителен",
+            "pathNotFound": "Путь не существует",
+            "notADirectory": "Не является каталогом",
+            "notReadable": "Путь недоступен для чтения",
+            "notWritable": "Путь недоступен для записи"
+        },
         "priorityTags": {
             "title": "Приоритетные теги",
             "description": "Настройте порядок приоритетов тегов для каждого типа моделей (например, character, concept, style(toon|toon_style)).",