fix: migration des onglets singleton → PaneTabManager au split

_buildGrid: quand on passe de single-pane à multi-pane, migre les onglets
du TabManager singleton vers le PaneTabManager du pane 0.
Sans ça, transferTab() ne trouvait pas l'onglet (il était resté dans le singleton).

_collapseToSingle: migration inverse (PaneTabManager → singleton)
quand on ferme le dernier split.

frontend/js/pane-manager.js

@@ -700,6 +700,30 @@ const PaneManager = {
     }
     this.activePaneId = 0;
     this._highlightActive();
+
+    // Migrate tabs from singleton TabManager to pane-0 PaneTabManager
+    // (the singleton managed all tabs in single-pane mode)
+    if (!savedPanes && window._singletonTabManager && window._singletonTabManager._tabs.length > 0) {
+      const stm = window._singletonTabManager;
+      const ptm = this.tabManagers[0];
+      if (ptm && ptm._tabs.length === 0) {
+        console.log('[PaneManager] migrating', stm._tabs.length, 'tabs from singleton to pane-0');
+        ptm._tabs = stm._tabs.map(t => ({ ...t }));
+        ptm._tabCache = { ...stm._tabCache };
+        ptm._activeTabId = stm._activeTabId;
+        ptm._previewTabId = stm._previewTabId;
+        // Clear singleton — it now delegates to PaneManager
+        stm._tabs = [];
+        stm._tabCache = {};
+        stm._activeTabId = null;
+        stm._previewTabId = null;
+        // Render the migrated tabs
+        setTimeout(() => {
+          ptm._renderTabs();
+          if (ptm._activeTabId) ptm.activate(ptm._activeTabId);
+        }, 100);
+      }
+    }
   },
 
   _createPane(idx, grid, reuseTabBar, reuseContent) {
@@ -901,6 +925,20 @@ const PaneManager = {
     if (content) { content.id = 'content-area'; wrapper.appendChild(content); }
     const tl = wrapper.querySelector('.tab-list');
     if (tl) tl.id = 'tab-list';
+
+    // Migrate tabs back to singleton TabManager
+    const ptm = this.tabManagers[0];
+    if (ptm && window._singletonTabManager) {
+      const stm = window._singletonTabManager;
+      stm._tabs = ptm._tabs.map(t => ({ ...t }));
+      stm._tabCache = { ...ptm._tabCache };
+      stm._activeTabId = ptm._activeTabId;
+      stm._previewTabId = ptm._previewTabId;
+      stm._tabBar = tabBar;
+      stm._tabList = tl;
+      stm._renderTabs();
+    }
+
     // Re-init pane-0 TabManager to point to restored DOM
     if (this.tabManagers[0]) {
       this.tabManagers[0]._tabBar = tabBar;