fix(maestro): cap and centre the spawn form's width

The form filled the full pane width, so on a 4K display every input
stretched thousands of pixels wide. Bound it to 60ch (in line with the
48ch/42ch precedents already in the type scale) and centre it with
margin auto; panes narrower than the cap still fill edge to edge.

Closes #92

web/src/styles.css

@@ -1215,6 +1215,9 @@ textarea {
 /* ---------- forms ---------- */
 
 .spawn-form {
+  width: 100%;
+  max-width: 60ch;
+  margin: 0 auto;
   padding: var(--sp-3);
   overflow: auto;
   display: flex;