Add initial version of Manual Invoice functionality with PDF generation and management

public/js/pages/ManualInvoice/ManualInvoice.css

@@ -28,10 +28,40 @@
 .invoice-preview-pane {
     flex: 1 1 auto;
     background-color: #525659;
-    padding: 2rem;
-    overflow-y: auto;
+    padding: 0;
+    overflow: hidden;
     display: flex;
     justify-content: center;
+    align-items: center;
+}
+
+.pdf-preview-container {
+    width: 100%;
+    height: 100%;
+    position: relative;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+}
+
+.pdf-preview-container object {
+    width: 100%;
+    height: 100%;
+}
+
+.pdf-loading {
+    text-align: center;
+    color: white;
+    padding: 2rem;
+}
+
+.pdf-loading i {
+    margin-bottom: 1rem;
+}
+
+.pdf-loading p {
+    font-size: 1.2rem;
+    margin-top: 1rem;
 }
 
 .info-bar {