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">

web/static/main.js

@@ -128,6 +128,7 @@ const solveProject  = (pid, preview) => api("POST", `/projects/${pid}/solve${pre
 const portsAndResolve = (pid, devId, body) => api("POST", `/projects/${pid}/devices/${devId}/ports-and-resolve`, body);
 const listSetupTemplates = () => api("GET", `/setup-templates`);
 const applyTemplate = (pid, body) => api("POST", `/projects/${pid}/apply-template`, body);
+const syncExport     = (pid) => api("POST", `/projects/${pid}/sync/export`, {});
 
 // ---------- DOM helpers ---------- //
 
@@ -2112,6 +2113,40 @@ function renderTemplatePreview(preview, templateIDStr) {
   `;
 }
 
+// ---------- export flow ---------- //
+
+let toastTimer = null;
+
+function showToast(kind, html, holdMs = 5000) {
+  const t = $("#toast");
+  t.className = "toast " + (kind || "");
+  t.innerHTML = html;
+  setHidden(t, false);
+  if (toastTimer) clearTimeout(toastTimer);
+  toastTimer = setTimeout(() => { setHidden(t, true); t.innerHTML = ""; }, holdMs);
+}
+
+async function exportCurrentProject() {
+  if (!state.active) { alert("Pick a project first"); return; }
+  const btn = $("#btn-export");
+  btn.disabled = true;
+  showToast("", "Exporting…", 30000);
+  try {
+    const res = await syncExport(state.active.id);
+    const url = res.url ?? "(no url)";
+    const count = res.element_count ?? 0;
+    showToast("ok",
+      `Exported ${count} elements → <a href="${escapeHtml(url)}" target="_blank" rel="noopener">${escapeHtml(url)}</a>`,
+      8000);
+  } catch (e) {
+    // Surface mxdrw unreachability or the upstream error verbatim.
+    const detail = typeof e.details === "object" ? JSON.stringify(e.details) : (e.details ?? "");
+    showToast("error", `Export failed: ${escapeHtml(e.message)}${detail ? ` (${escapeHtml(String(detail))})` : ""}`, 12000);
+  } finally {
+    btn.disabled = false;
+  }
+}
+
 // ---------- boot ---------- //
 
 async function boot() {
@@ -2133,6 +2168,7 @@ async function boot() {
   });
   $("#btn-solve").addEventListener("click", openSolveModal);
   $("#btn-apply-template").addEventListener("click", openApplyTemplateModal);
+  $("#btn-export").addEventListener("click", exportCurrentProject);
 
   $("#project-select").addEventListener("change", (e) => {
     const v = /** @type {HTMLSelectElement} */ (e.target).value;

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