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-TW.json

@@ -608,6 +608,22 @@
                 "controlnet": "ControlNet 路徑"
             }
         },
+        "directoryPicker": {
+            "title": "瀏覽資料夾",
+            "selectFolder": "選擇此資料夾",
+            "goUp": "上一層",
+            "pathPlaceholder": "輸入路徑...",
+            "go": "前往",
+            "emptyFolder": "沒有子資料夾",
+            "loadError": "目錄載入失敗"
+        },
+        "pathValidation": {
+            "valid": "路徑有效",
+            "pathNotFound": "路徑不存在",
+            "notADirectory": "不是目錄",
+            "notReadable": "路徑無法讀取",
+            "notWritable": "路徑無法寫入"
+        },
         "priorityTags": {
             "title": "優先標籤",
             "description": "為每種模型類型自訂標籤的優先順序 (例如: character, concept, style(toon|toon_style))",