fix(sync): stop sync loop; add link warning and linked-devices table

- Await restorePersistedTunnels so applyingSync stays true for full restore,
  preventing saveState() during tunnel startup from pushing back and looping
- Debounce onRemoteUpdate (600ms) and skip apply when state unchanged
  (canonical fingerprint) to avoid redundant restarts
- Add confirm modal before "Link device": warns that current state will be
  replaced and suggests creating a backup first
- Add Linked devices table when linked: show This device ID (writerKey) and
  Sync group ID (discoveryKey); getSyncStatus returns deviceId and syncGroupId

extension/dashboard/pages/sync.js

@@ -1,6 +1,6 @@
 /**
  * Sync page — device linking via autopass; create invite, pair with invite, show status.
- * Depends: core/utils.js ($), core/messaging.js (sendToNative), ui/toast.js (showToast)
+ * Depends: core/utils.js ($), ui/modal.js (openModal, closeModal, showModalError), ui/toast.js (showToast)
  */
 
 /**
@@ -41,12 +41,25 @@ function updateSyncStatus() {
           if (btnCopy) btnCopy.style.display = 'none';
           if (inviteDisplay) inviteDisplay.style.display = 'none';
         }
+        const card = $('syncLinkedDevicesCard');
+        const tbody = $('syncLinkedDevicesTable');
+        if (card) card.style.display = 'block';
+        if (tbody) {
+          const deviceId = response.deviceId || '—';
+          const syncGroupId = response.syncGroupId || '—';
+          tbody.innerHTML = '<tr><td>This device</td><td class="mono" style="font-size:12px;">' + escapeHtml(deviceId) + '</td></tr>' +
+            '<tr><td>Sync group</td><td class="mono" style="font-size:12px;">' + escapeHtml(syncGroupId) + '</td></tr>';
+        }
       } else {
         if (dot) dot.style.background = 'var(--text4)';
         text.textContent = 'Not linked';
         if (lastSynced) lastSynced.textContent = '';
         if (btnCopy) btnCopy.style.display = 'none';
         if (inviteDisplay) inviteDisplay.style.display = 'none';
+        const card = $('syncLinkedDevicesCard');
+        const tbody = $('syncLinkedDevicesTable');
+        if (card) card.style.display = 'none';
+        if (tbody) tbody.innerHTML = '<tr><td colspan="2" class="empty-cell">Not linked</td></tr>';
       }
     }
   );
@@ -103,14 +116,24 @@ function setupSyncEvents() {
       showToast('Paste an invite first', 'error');
       return;
     }
-    if (btnPair.disabled) return;
-    btnPair.disabled = true;
-    btnPair.textContent = 'Linking…';
+    const errEl = $('linkDeviceError');
+    if (errEl) { errEl.style.display = 'none'; errEl.textContent = ''; }
+    openModal('modal-linkDevice');
+  });
+
+  $('linkDeviceConfirm')?.addEventListener('click', () => {
+    const invite = inviteInput?.value?.trim();
+    const errEl = $('linkDeviceError');
+    if (!invite) {
+      if (errEl) { errEl.textContent = 'Paste an invite in the field below first.'; errEl.style.display = 'block'; }
+      return;
+    }
+    closeModal('modal-linkDevice');
+    if (btnPair) { btnPair.disabled = true; btnPair.textContent = 'Linking…'; }
     chrome.runtime.sendMessage(
       { target: 'holesail-native', action: 'send', payload: { type: 'pairWithInvite', payload: { invite } } },
       (response) => {
-        btnPair.disabled = false;
-        btnPair.textContent = 'Link device';
+        if (btnPair) { btnPair.disabled = false; btnPair.textContent = 'Link device'; }
         if (chrome.runtime.lastError) {
           showToast('Failed: ' + (chrome.runtime.lastError.message || 'unknown'), 'error');
           return;