Adopt shared WebUI layout primitives

webui/src/styles/postbox.css

@@ -420,15 +420,6 @@
   width: min(900px, calc(100vw - 32px));
 }
 
-.postbox-form-grid {
-  display: grid;
-  gap: 14px;
-}
-
-.postbox-form-grid.two-columns {
-  grid-template-columns: repeat(2, minmax(0, 1fr));
-}
-
 .postbox-form-grid input,
 .postbox-form-grid select {
   width: 100%;
@@ -600,9 +591,7 @@
 }
 
 .postbox-compose-grid {
-  display: grid;
   grid-template-columns: minmax(0, 2fr) minmax(160px, 1fr);
-  gap: 12px;
 }
 
 .postbox-compose-wide {
@@ -755,7 +744,6 @@
 }
 
 @media (max-width: 640px) {
-  .postbox-form-grid.two-columns,
   .postbox-provenance dl,
   .postbox-admin-properties {
     grid-template-columns: 1fr;
@@ -770,7 +758,4 @@
     flex-direction: column;
   }
 
-  .postbox-compose-grid {
-    grid-template-columns: 1fr;
-  }
 }