make ffmpeg command holder fill space

src/App.svelte

@@ -265,6 +265,8 @@
 
   .command {
     grid-area: cmd;
+    display: flex;
+    flex-direction: column;
   }
 
   .inputs {
@@ -338,6 +340,7 @@
     display: flex;
     align-items: center;
     margin: 10px 0px;
+    flex: 1;
   }
 
   .actual-command {
@@ -347,6 +350,7 @@
     flex: 1;
     font: inherit;
     padding: 5px;
+    height: 100%;
   }
   .section-header {
     display: flex;