refactor(webdav): Use ResolvePath instead of JoinPath (#9344)

- Changed the path concatenation method between `reqPath` and `src` and `dst` to use `ResolvePath`
- Updated the implementation of path handling in multiple functions
- Improved the consistency and reliability of path resolution

server/webdav/path.go

@@ -0,0 +1,22 @@
+package webdav
+
+import (
+	"path"
+	"strings"
+
+	"github.com/alist-org/alist/v3/internal/model"
+	"github.com/alist-org/alist/v3/pkg/utils"
+)
+
+// ResolvePath normalizes the provided raw path and resolves it against the user's base path
+// before delegating to the user-aware JoinPath permission checks.
+func ResolvePath(user *model.User, raw string) (string, error) {
+	cleaned := utils.FixAndCleanPath(raw)
+	basePath := utils.FixAndCleanPath(user.BasePath)
+
+	if cleaned != "/" && basePath != "/" && !utils.IsSubPath(basePath, cleaned) {
+		cleaned = path.Join(basePath, strings.TrimPrefix(cleaned, "/"))
+	}
+
+	return user.JoinPath(cleaned)
+}