feat: finalisation split view #75 — boutons, nav clavier, palette, ROADMAP

Ajouts:
- C3: Boutons Split ⊞→ ⊞↓ dans la barre d'onglets (visibles au survol)
- C2: Ctrl+Alt+←/→/↑/↓ navigation entre panneaux
- C2: Ctrl+Shift+Alt+W fermer tous les autres panneaux
- H1: 5 commandes palette (Split R/D, Close Pane, Focus Next/Prev)
- ROADMAP.md: section 75 mise à jour avec statut 80% complété

CSS: .tab-split-buttons, .tab-split-btn avec opacity hover

frontend/style.css

@@ -6526,6 +6526,41 @@ body.popup-mode .content-area {
   scrollbar-width: thin;
 }
 
+/* Split buttons in tab bar */
+.tab-split-buttons {
+  display: flex;
+  align-items: center;
+  gap: 2px;
+  padding: 0 4px;
+  flex-shrink: 0;
+  opacity: 0.3;
+  transition: opacity 150ms ease;
+}
+
+.tab-bar:hover .tab-split-buttons,
+.tab-split-buttons:hover {
+  opacity: 1;
+}
+
+.tab-split-btn {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 24px;
+  height: 24px;
+  border: none;
+  border-radius: 4px;
+  background: transparent;
+  color: var(--text-muted);
+  cursor: pointer;
+  transition: background 100ms ease, color 100ms ease;
+}
+
+.tab-split-btn:hover {
+  background: var(--bg-hover);
+  color: var(--accent);
+}
+
 .tab-list::-webkit-scrollbar {
   height: 3px;
 }