fix(ui): center create-stack label and drop duplicate plus on mesh CTA (#1310)

The Create Stack button left a redundant `mr-2` on its Plus icon. The
Button base already applies `gap-2`, so the margin doubled the spacing
and pushed the centered icon-plus-label block right of center. Removing
`mr-2` restores the single correct gap.

The meshed-state routing CTA rendered both a Plus icon (via ctaIconFor)
and a literal "+ " prefix in the text, showing two plus glyphs. Dropped
the literal prefix so only the icon renders.

frontend/src/components/EditorLayout.tsx

@@ -315,7 +315,7 @@ export default function EditorLayout() {
         className="rounded-lg w-full"
         onClick={() => openCreateDialog('empty')}
       >
-        <Plus className="w-4 h-4 mr-2" />
+        <Plus className="w-4 h-4" />
         Create Stack
       </Button>
       <CreateStackDialog