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/de.json

@@ -608,6 +608,22 @@
                 "controlnet": "ControlNet-Pfade"
             }
         },
+        "directoryPicker": {
+            "title": "Ordner durchsuchen",
+            "selectFolder": "Diesen Ordner auswählen",
+            "goUp": "Nach oben",
+            "pathPlaceholder": "Pfad eingeben...",
+            "go": "Los",
+            "emptyFolder": "Keine Unterordner",
+            "loadError": "Verzeichnis konnte nicht geladen werden"
+        },
+        "pathValidation": {
+            "valid": "Pfad ist gültig",
+            "pathNotFound": "Pfad existiert nicht",
+            "notADirectory": "Kein Verzeichnis",
+            "notReadable": "Pfad ist nicht lesbar",
+            "notWritable": "Pfad ist nicht beschreibbar"
+        },
         "priorityTags": {
             "title": "Prioritäts-Tags",
             "description": "Passen Sie die Tag-Prioritätsreihenfolge für jeden Modelltyp an (z. B. character, concept, style(toon|toon_style))",