fix(settings): reject checkpoints↔unet path overlap in extra folder paths with inline error UI

Changes:
- Backend: _validate_folder_paths() now checks checkpoints↔unet overlap
  within the same library using os.path.realpath() for symlink resolution
- Backend: set() calls _validate_folder_paths() for both folder_paths and
  extra_folder_paths before writing
- Backend: extracted _normalize_path_set() helper to eliminate duplicated
  normalization logic
- Frontend: inline error display with red border + error message below the
  conflicting input, no save triggered
- Frontend: path normalization (strip trailing slash, lowercase) in pre-check
  to reduce false negatives vs backend realpath
- Frontend: asymmetric error UX — message only on the user-edited side,
  red border on the pre-existing conflict side
- CSS: has-error styles with hardcoded rgba fallback for older browsers
- i18n: checkpointUnetOverlap + checkpointUnetOverlapInline keys added to
  all 10 locale files

locales/ja.json

@@ -505,7 +505,9 @@
             "saveSuccess": "追加フォルダーパスを更新しました。変更を適用するには再起動が必要です。",
             "saveError": "追加フォルダーパスの更新に失敗しました: {message}",
             "validation": {
-                "duplicatePath": "このパスはすでに設定されています"
+                "duplicatePath": "このパスはすでに設定されています",
+                "checkpointUnetOverlap": "checkpoints と diffusion models に同じパスは使用できません：{paths}",
+                "checkpointUnetOverlapInline": "このパスは別のモデルタイプですでに使用されています。checkpoints と diffusion models には別々のフォルダを使用してください。"
             }
         },
         "priorityTags": {