fix: dragstart singleton TabManager — JSON au lieu de String(idx)

Cause: le TabManager singleton (single-pane) écrasait text/plain avec String(idx)
→ getData retournait '1' au lieu du JSON {paneId,tabId,index}
→ le drop handler PaneManager échouait sur 'missing paneId/tabId'

Fix:
- singleton dragstart: JSON.stringify({paneId:0, tabId, index}) + effectAllowed
- singleton drop: parse JSON, fallback legacy parseInt pour rétrocompatibilité
- dragover/drop inchangés (indicateurs visuels)

frontend/js/ui.js

@@ -2267,9 +2267,11 @@ export const TabManager = {
         this._showTabContextMenu(e.clientX, e.clientY, tab.id);
       });
 
-      // Drag and drop
+      // Drag and drop — JSON format for cross-pane compatibility
       el.addEventListener("dragstart", (e) => {
-        e.dataTransfer.setData("text/plain", String(idx));
+        const payload = JSON.stringify({ paneId: 0, tabId: tab.id, index: idx });
+        e.dataTransfer.setData("text/plain", payload);
+        e.dataTransfer.effectAllowed = "move";
         el.classList.add("dragging");
       });
       el.addEventListener("dragend", () => {
@@ -2293,13 +2295,29 @@ export const TabManager = {
       el.addEventListener("drop", (e) => {
         e.preventDefault();
         document.querySelectorAll(".tab-drop-indicator").forEach(d => d.remove());
-        const fromIdx = parseInt(e.dataTransfer.getData("text/plain"));
-        const rect = el.getBoundingClientRect();
-        const mid = rect.left + rect.width / 2;
-        const toIdx = e.clientX < mid ? idx : idx + 1;
-        if (fromIdx !== toIdx && fromIdx !== toIdx - 1) {
-          this.moveTab(fromIdx, toIdx);
+        const raw = e.dataTransfer.getData("text/plain");
+        if (!raw) return;
+        let dragData;
+        try { dragData = JSON.parse(raw); } catch (e2) {
+          // Legacy format: plain index string
+          const fromIdx = parseInt(raw);
+          if (isNaN(fromIdx)) return;
+          const rect = el.getBoundingClientRect();
+          const mid = rect.left + rect.width / 2;
+          const toIdx = e.clientX < mid ? idx : idx + 1;
+          if (fromIdx !== toIdx && fromIdx !== toIdx - 1)
+            this.moveTab(fromIdx, toIdx);
+          return;
         }
+        if (dragData.paneId === 0) {
+          // Same pane — reorder
+          const rect = el.getBoundingClientRect();
+          const mid = rect.left + rect.width / 2;
+          const toIdx = e.clientX < mid ? idx : idx + 1;
+          if (dragData.index !== toIdx && dragData.index !== toIdx - 1)
+            this.moveTab(dragData.index, toIdx);
+        }
+        // Cross-pane transfers handled by PaneManager's container drop zone
       });
 
       this._tabList.appendChild(el);