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/ko.json

@@ -608,6 +608,22 @@
                 "controlnet": "ControlNet 경로"
             }
         },
+        "directoryPicker": {
+            "title": "폴더 찾아보기",
+            "selectFolder": "이 폴더 선택",
+            "goUp": "위로",
+            "pathPlaceholder": "경로 입력...",
+            "go": "이동",
+            "emptyFolder": "하위 폴더 없음",
+            "loadError": "디렉터리를 불러오지 못했습니다"
+        },
+        "pathValidation": {
+            "valid": "유효한 경로입니다",
+            "pathNotFound": "경로가 존재하지 않습니다",
+            "notADirectory": "디렉터리가 아닙니다",
+            "notReadable": "경로를 읽을 수 없습니다",
+            "notWritable": "경로에 쓸 수 없습니다"
+        },
         "priorityTags": {
             "title": "우선순위 태그",
             "description": "모델 유형별 태그 우선순위를 사용자 지정합니다(예: character, concept, style(toon|toon_style)).",