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.

py/utils/directory_browser.py

@@ -0,0 +1,152 @@
+"""Shared directory-browsing logic for HTTP directory pickers."""
+
+from __future__ import annotations
+
+import os
+from pathlib import Path
+from typing import Any, Dict, Tuple
+
+# Virtual path token for the Windows drive list. Browsing up from a drive
+# root (e.g. C:\) lands here so users can switch drives without typing a
+# path. Only meaningful on Windows; elsewhere it falls through to normal
+# path handling and fails the existence check.
+WINDOWS_DRIVES_TOKEN = "__drives__"
+
+_IMAGE_EXTENSIONS = {
+    ".jpg",
+    ".jpeg",
+    ".png",
+    ".gif",
+    ".webp",
+    ".bmp",
+    ".tiff",
+    ".tif",
+}
+
+
+def browse_directory(directory_path: str) -> Tuple[Dict[str, Any], int]:
+    """Browse a directory and return (payload, http_status).
+
+    The payload shape matches the JSON responses historically produced by
+    ``BatchImportHandler.browse_directory``: on success a dict with
+    ``success``, ``current_path``, ``parent_path``, ``directories``,
+    ``image_files``, ``image_count`` and ``directory_count``; on failure a
+    ``{"success": False, "error": ...}`` dict with a 400/403/404/500 status.
+    """
+    if os.name == "nt" and directory_path == WINDOWS_DRIVES_TOKEN:
+        return _windows_drives_payload(), 200
+
+    # Default to the user's home directory. The frontend previously
+    # sent "/" as the initial path, which is POSIX-only: on Windows it
+    # resolves to the current drive root and then fails the access
+    # check below.
+    if not directory_path:
+        path = Path.home()
+    else:
+        path = Path(directory_path).expanduser().resolve()
+
+    # Access check: browsing intentionally covers the whole server
+    # filesystem (the server operator browses their own machine). On
+    # POSIX every absolute path is under "/", but Path("/") has no
+    # drive letter on Windows and can never anchor a drive-qualified
+    # path in relative_to(), so test for a drive there instead.
+    if os.name == "nt":
+        is_allowed = bool(path.drive)
+    else:
+        is_allowed = path.is_absolute()
+
+    if not is_allowed:
+        return {"success": False, "error": "Access denied to this directory"}, 403
+
+    if not path.exists():
+        return {"success": False, "error": "Directory does not exist"}, 404
+
+    if not path.is_dir():
+        return {"success": False, "error": "Path is not a directory"}, 400
+
+    directories = []
+    image_files = []
+
+    try:
+        for item in path.iterdir():
+            try:
+                if item.is_dir():
+                    # Skip hidden directories and common system folders
+                    if not item.name.startswith(".") and item.name not in [
+                        "__pycache__",
+                        "node_modules",
+                    ]:
+                        directories.append(
+                            {
+                                "name": item.name,
+                                "path": str(item),
+                                "is_parent": False,
+                            }
+                        )
+                elif item.is_file() and item.suffix.lower() in _IMAGE_EXTENSIONS:
+                    image_files.append(
+                        {
+                            "name": item.name,
+                            "path": str(item),
+                            "size": item.stat().st_size,
+                        }
+                    )
+            except (PermissionError, OSError):
+                # Skip files/directories we can't access
+                continue
+
+        directories.sort(key=lambda x: x["name"].lower())
+        image_files.sort(key=lambda x: x["name"].lower())
+
+        # Parent directory. A filesystem root is its own parent
+        # (parent == path): POSIX "/" gets no parent, while a Windows
+        # drive root (C:\) links up to the virtual drive list so users
+        # can switch drives. The previous str(path) != str(path.root)
+        # check misfired on Windows, where a drive root's parent is
+        # itself, producing an infinite self-loop.
+        if path.parent == path:
+            parent_path = WINDOWS_DRIVES_TOKEN if os.name == "nt" else None
+        else:
+            parent_path = str(path.parent)
+
+        return (
+            {
+                "success": True,
+                "current_path": str(path),
+                "parent_path": parent_path,
+                "directories": directories,
+                "image_files": image_files,
+                "image_count": len(image_files),
+                "directory_count": len(directories),
+            },
+            200,
+        )
+
+    except PermissionError:
+        return {"success": False, "error": "Permission denied"}, 403
+    except OSError as exc:
+        return {"success": False, "error": f"Error reading directory: {str(exc)}"}, 500
+
+
+def _windows_drives_payload() -> Dict[str, Any]:
+    """List available drive letters as a virtual directory (Windows only)."""
+    try:
+        drives = os.listdrives()
+    except AttributeError:  # Python < 3.12
+        drives = [
+            f"{letter}:\\"
+            for letter in "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+            if os.path.exists(f"{letter}:\\")
+        ]
+    directories = [{"name": drive, "path": drive, "is_parent": False} for drive in drives]
+    return {
+        "success": True,
+        # Empty current_path marks the virtual level; the frontend
+        # disables folder selection there.
+        "current_path": "",
+        "parent_path": None,
+        "directories": directories,
+        "image_files": [],
+        "image_count": 0,
+        "directory_count": len(directories),
+    }