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/style.css

@@ -236,6 +236,24 @@ body {
   filter: drop-shadow(0 0 4px var(--accent));
 }
 
+/* Header toast — slice 8 export feedback */
+.toast {
+  display: inline-block;
+  margin-left: 12px;
+  font-size: 13px;
+  padding: 4px 10px;
+  border-radius: var(--radius);
+  background: var(--surface-2);
+  color: var(--text);
+  max-width: 420px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+.toast.ok    { background: #e8f5e9; color: #1b5e20; }
+.toast.error { background: #fdecea; color: #911313; }
+.toast a { color: inherit; text-decoration: underline; }
+
 /* IO markers — diamonds. Power-by-convention, so the default fill is
    the Power cable_type colour (#e03131). Rotated 45° rect is the
    easiest way to draw a diamond that still hit-tests at the rotated