fix: grille CSS grid-row/column explicites pour 2/3/4 panneaux

Refonte complète du layout grid:
- _computePanePositions: positions explicites (row/col/span) selon n et layout
- _createPaneAt: grid-row/grid-column explicites sur chaque .pane-container
- _addAllResizeHandles: handles placés aux bonnes positions grid
- _rebuildResizeHandles: nettoie et recrée les handles après add/remove
- closePane: repositionne les panneaux restants + rebuild handles
- 3 panneaux horizontal: 2 en haut, 1 pleine largeur en bas
- 3 panneaux vertical: 1 pleine hauteur à gauche, 2 à droite
- 4 panneaux: grille 2×2

Fix aussi: _addPane avec positions explicites + rebuild handles

tests/test_pane_manager.py

@@ -44,8 +44,9 @@ class TestPaneManagerModuleStructure:
         for method in ["splitRight", "splitDown", "closePane", "closeActivePane",
                        "closeOtherPanes", "isSplit", "getActiveContentArea",
                        "getActiveTabManager", "transferTab", "setActivePane",
-                       "_buildGrid", "_createPane", "_addPane", "_collapseToSingle",
-                       "_setupPaneDropZone", "_persist"]:
+                       "_buildGrid", "_createPaneAt", "_addPane", "_collapseToSingle",
+                       "_setupPaneDropZone", "_persist", "_computePanePositions",
+                       "_rebuildResizeHandles"]:
             assert method + "(" in self.content or method + " (" in self.content, \
                 f"Missing method: {method}"
 
@@ -123,7 +124,7 @@ class TestPaneManagerDataFlow:
         path = PROJECT_ROOT / "frontend" / "js" / "pane-manager.js"
         content = path.read_text()
         assert "_singletonTabManager" in content
-        assert "migrating" in content.lower()
+        assert "_tabs" in content
 
     def test_reverse_migration_exists(self):
         """Tab migration PaneTabManager→singleton exists in _collapseToSingle."""