Add one-off next check number override

Pencil button next to "Next check: X" in the header opens a modal
with a warning about sequence gaps/duplicates. Saves via new
PUT /api/account/:id/check-no endpoint (sets current_check_no = next - 1).

public/css/style.css

@@ -64,6 +64,26 @@ header {
   line-height: 1.4;
 }
 .btn-header-icon:hover { background: rgba(255,255,255,0.28); }
+.btn-header-inline {
+  background: none;
+  border: none;
+  color: rgba(255,255,255,0.5);
+  font-size: 11px;
+  cursor: pointer;
+  padding: 0 0 0 4px;
+  line-height: 1;
+  vertical-align: middle;
+}
+.btn-header-inline:hover { color: #fff; }
+.warning-box {
+  background: #fef9c3;
+  border: 1px solid #ca8a04;
+  border-radius: 4px;
+  padding: 10px 12px;
+  font-size: 12px;
+  color: #713f12;
+  line-height: 1.5;
+}
 
 /* ── Toolbar ── */
 .toolbar {