feat(ui): wire Export button — POST /sync/export + toast

Export button is no longer disabled. On click it POSTs to the export
endpoint and shows a toast next to the button:
  ✓ Exported · open in mxdrw   (with viewer URL)
  ✗ Export failed — <detail>

web/static/index.html

@@ -22,9 +22,8 @@
     <div class="topbar-spacer"></div>
     <button type="button" id="btn-apply-template" class="btn">Apply template…</button>
     <button type="button" id="btn-solve" class="btn btn-primary">Solve</button>
-    <button type="button" id="btn-export" class="btn" disabled title="Slice 8">
-      Export
-    </button>
+    <button type="button" id="btn-export" class="btn">Export</button>
+    <span id="toast" class="toast" hidden></span>
   </header>
 
   <main class="layout">