Margin table: say "Covered" rather than $0.00 for break-even

Same fix already applied to the capacity cards, missed here. A zero break-even
means the block's cost is fully recovered and any further sale is upside;
printing "$0.00" is technically true and reads like a rendering bug. The demo
book has three such blocks, so it was visible on every screenshot.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/web/src/pages/Margin.tsx

@@ -123,10 +123,20 @@ export function Margin() {
                     <td className="nums px-4 py-3 text-right">
                       {moneyExact(block.costPerGpuHourCents)}
                     </td>
-                    <td className="nums px-4 py-3 text-right sm:px-5">
-                      {block.breakEvenPriceCents == null
-                        ? '—'
-                        : moneyExact(block.breakEvenPriceCents)}
+                    {/*
+                      A zero break-even means the block's cost is already
+                      covered, so any further sale is upside. Printing "$0.00"
+                      is technically true and reads like a bug — the same fix
+                      already applied on the capacity cards.
+                    */}
+                    <td className="px-4 py-3 text-right sm:px-5">
+                      {block.breakEvenPriceCents == null ? (
+                        <span className="text-muted">Sold out</span>
+                      ) : block.breakEvenPriceCents === 0 ? (
+                        <span className="text-positive">Covered</span>
+                      ) : (
+                        <span className="nums">{moneyExact(block.breakEvenPriceCents)}</span>
+                      )}
                     </td>
                   </tr>
                 ))}