feat(layout): add preview PDF button to layout editor

Generates a 3-up PDF with dummy check data (no real checks touched) so the
layout can be proofed without printing live checks.

Closes #12

public/index.html

@@ -886,7 +886,8 @@
           </div>
         </div>
         <div id="layout-save-status" style="font-size:11px;color:var(--text-muted);min-width:56px"></div>
-        <div style="margin-left:auto">
+        <div style="margin-left:auto;display:flex;gap:8px;align-items:center">
+          <button id="btn-layout-preview" class="btn-secondary btn-sm">⎙ Preview PDF</button>
           <button id="btn-layout-reset" class="btn-secondary btn-sm" data-admin-only>↺ Reset to Default</button>
         </div>
       </div>