further bug fixes when using context path (#1475)

Co-authored-by: Anthony Stirling <77850077+Frooodle@users.noreply.github.com>

src/main/resources/templates/convert/pdf-to-book.html

@@ -14,7 +14,7 @@
           <div class="row justify-content-center">
             <div class="col-md-6">
               <h2 th:text="#{PDFToBook.header}"></h2>
-              <form method="post" enctype="multipart/form-data" th:action="@{api/v1/convert/pdf/book}">
+              <form method="post" enctype="multipart/form-data" th:action="@{'/api/v1/convert/pdf/book'}">
                 <div th:replace="~{fragments/common :: fileSelector(name='fileInput', multiple=false, accept='application/pdf')}"></div>
                 <div class="mb-3">
                   <label th:text="#{PDFToBook.selectText.1}"></label>