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

static/css/components/modal/settings-modal.css

@@ -1562,6 +1562,29 @@ input:checked + .toggle-slider:before {
     box-shadow: 0 0 0 2px rgba(var(--lora-accent-rgb, 79, 70, 229), 0.1);
 }
 
+.extra-folder-path-row .path-controls .extra-folder-path-input.has-error {
+    border-color: var(--lora-error);
+    background-color: rgba(220, 53, 69, 0.08);
+    background-color: rgba(from var(--lora-error) r g b / 0.08);
+}
+
+.extra-folder-path-row .path-controls .extra-folder-path-input.has-error:focus {
+    box-shadow: 0 0 0 2px rgba(220, 53, 69, 0.15);
+    box-shadow: 0 0 0 2px rgba(from var(--lora-error) r g b / 0.15);
+}
+
+.extra-folder-path-error {
+    color: var(--lora-error);
+    font-size: 0.8em;
+    margin-top: 4px;
+    line-height: 1.4;
+    display: none;
+}
+
+.extra-folder-path-error.visible {
+    display: block;
+}
+
 .extra-folder-path-row .path-controls .remove-path-btn {
     width: 32px;
     height: 32px;