fix(post form): align desktop post form to blotter left edge

Match the blotter's centered 468px column so the desktop post form
shares its left edge instead of being centered within its own width.

src/components/post-form/post-form.module.css

@@ -10,7 +10,6 @@
 .postFormDesktop {
   font-size: var(--post-form-toggle-font-size);
   font-weight: var(--post-form-toggle-font-weight);
-  text-align: center;
 }
 
 .postFormMobile {
@@ -170,6 +169,18 @@
   .postFormMobile {
     display: none;
   }
+
+  .postFormDesktop {
+    width: 468px;
+    margin-left: auto;
+    margin-right: auto;
+    text-align: left;
+  }
+
+  .postFormTable {
+    margin-left: 0;
+    margin-right: 0;
+  }
 }
 
 .uploadButton button {