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/ui/routing-node-card.tsx

@@ -544,7 +544,7 @@ function emptyStateCopy(state: RoutingNodeState, name: string, offlineReason?: s
             return {
                 headline: 'No services routed yet.',
                 sub: 'Mesh is on. Opt a stack in to start publishing aliases.',
-                cta: '+ Add stack to mesh',
+                cta: 'Add stack to mesh',
             };
         case 'degraded':
             return {