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/zh-CN.json

@@ -608,6 +608,22 @@
                 "controlnet": "ControlNet 路径"
             }
         },
+        "directoryPicker": {
+            "title": "浏览文件夹",
+            "selectFolder": "选择此文件夹",
+            "goUp": "上级目录",
+            "pathPlaceholder": "输入路径...",
+            "go": "跳转",
+            "emptyFolder": "没有子文件夹",
+            "loadError": "目录加载失败"
+        },
+        "pathValidation": {
+            "valid": "路径有效",
+            "pathNotFound": "路径不存在",
+            "notADirectory": "不是一个目录",
+            "notReadable": "路径不可读",
+            "notWritable": "路径不可写"
+        },
         "priorityTags": {
             "title": "优先标签",
             "description": "为每种模型类型自定义标签优先级顺序 (例如: character, concept, style(toon|toon_style))",