feat: split view multi-panneaux (#75) — PaneManager intégré avec split right/down, menu contextuel, raccourcis clavier, CSS responsive et tests

- PaneManager: splitRight/splitDown avec 1-4 panneaux, persistance localStorage, resize
- Viewer/TabManager pane-aware: fonctions getContentArea/getTabBar/getTabList
- Menu contextuel onglets: actions Diviser à droite / Diviser en bas / Fermer le panneau
- Raccourcis clavier: Ctrl+Alt+\ (split right), Ctrl+Alt+Shift+\ (split down), Ctrl+Alt+W (fermer panneau)
- CSS pane-grid: grille, poignées resize, bordure active pane, responsive (désactivé <768px)
- Tests: 22 nouveaux tests unitaires + régression (307 total, 100% green)
- Déployé sur Docker test (localhost:2021)

frontend/style.css

@@ -1247,6 +1247,93 @@ select {
   overflow: hidden;
 }
 
+/* --- Multi-Pane Grid (Split View #75) --- */
+.pane-grid {
+  flex: 1;
+  display: grid;
+  overflow: hidden;
+  min-width: 0;
+  min-height: 0;
+}
+
+.pane-container {
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+  min-width: 0;
+  min-height: 0;
+  border: 2px solid transparent;
+  transition: border-color 150ms ease;
+}
+
+.pane-container.pane-active {
+  border-color: var(--accent);
+}
+
+.pane-container .tab-bar.pane-tab-bar {
+  flex-shrink: 0;
+}
+
+.pane-container .content-area.pane-content {
+  flex: 1;
+  overflow-y: auto;
+  overflow-x: hidden;
+  padding: clamp(12px, 2vw, 24px) clamp(12px, 3vw, 24px) 40px;
+}
+
+.pane-resize-handle {
+  width: 4px;
+  background: var(--border);
+  transition: background 150ms ease;
+  position: relative;
+  z-index: 2;
+  flex-shrink: 0;
+}
+
+.pane-resize-handle:hover {
+  background: var(--accent);
+}
+
+.pane-resize-handle::after {
+  content: '';
+  position: absolute;
+  top: 10%;
+  bottom: 10%;
+  left: 50%;
+  transform: translateX(-50%);
+  width: 2px;
+  background: var(--accent);
+  opacity: 0;
+  transition: opacity 150ms ease;
+}
+
+.pane-resize-handle:hover::after {
+  opacity: 1;
+}
+
+/* Pane active state */
+.pane-container.pane-active {
+  outline: none;
+  border: 2px solid var(--accent);
+}
+
+/* Responsive: disable split view below 768px */
+@media (max-width: 768px) {
+  .pane-grid {
+    display: flex !important;
+    flex-direction: column !important;
+  }
+  .pane-grid .pane-container:not(:first-child) {
+    display: none;
+  }
+  .pane-grid .pane-resize-handle {
+    display: none;
+  }
+  .pane-container.pane-active {
+    border-color: transparent;
+  }
+}
+
 /* --- Content Area --- */
 .content-area {
   flex: 1;