feat(eoi): signed-EOI hero + send-signed-copy; fix search dropdown z-order

- EOI tab: when an EOI is already signed and none is in flight, lead with a
  SignedEoiCard (preview + download + send-to-client) instead of the big
  "Generate EOI" empty state; quiet "Generate new EOI" remains for re-issue
- history rows + hero gain a "Send to client" action — POST
  /api/v1/documents/[id]/send-signed-copy emails the deal's client the
  finalized signed PDF (sendSignedCopyToClient reuses sendSigningCompleted),
  guarded by a confirm
- topbar: header gets z-30 so the global search dropdown paints above page
  content (charts/tables were bleeding through — header + main are sibling
  normal-flow boxes, so the dropdown's own z-50 couldn't win cross-context).
  Stays below the z-50 modal tier.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/layout/topbar.tsx

@@ -51,7 +51,13 @@ export function Topbar({ ports, user, leadingSlot }: TopbarProps) {
     // Sarah Doe"). Detail pages register their parent via
     // `useBreadcrumbHint` so the label is entity-aware; everything else
     // is URL-derived. See src/hooks/use-smart-back.ts.
-    <header className="relative grid h-14 grid-cols-[auto_1fr_auto] items-center border-b border-border bg-background gap-3 px-4 shrink-0">
+    // `z-30` makes the header a stacking context that sits ABOVE the page
+    // content in <main>. Without it the search dropdown (absolute, z-50 within
+    // the header) still paints under main's charts/tables: header + main are
+    // sibling normal-flow boxes, so main paints later and wins regardless of
+    // the dropdown's own z. Kept below the modal/overlay tier (z-50) so
+    // dialogs still cover the bar.
+    <header className="relative z-30 grid h-14 grid-cols-[auto_1fr_auto] items-center border-b border-border bg-background gap-3 px-4 shrink-0">
       {/* LEFT: optional sidebar trigger (tablet) + smart back button.
           Hard-capped width so the column never extends into the
           absolutely-positioned search bar's footprint. The cap is