feat(dragdrop): drag & drop complet de fichiers/dossiers & intégration IA (#89)

frontend/style.css

@@ -1124,6 +1124,93 @@ select {
   outline-offset: -2px;
 }
 
+/* Global Drag & Drop Overlay (#89) */
+.obsigate-drop-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 99999;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  opacity: 0;
+  pointer-events: none;
+  transition: opacity 0.2s cubic-bezier(0.16, 1, 0.3, 1);
+}
+
+.obsigate-drop-overlay.active {
+  opacity: 1;
+  pointer-events: auto;
+}
+
+.obsigate-drop-backdrop {
+  position: absolute;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.65);
+  backdrop-filter: blur(4px);
+}
+
+.obsigate-drop-modal {
+  position: relative;
+  z-index: 1;
+  background: var(--bg-primary, #1e1e2e);
+  border: 1px solid var(--border-color, #313244);
+  border-radius: 12px;
+  padding: 24px;
+  width: min(600px, 90vw);
+  box-shadow: 0 16px 32px rgba(0, 0, 0, 0.4);
+  text-align: center;
+}
+
+.obsigate-drop-title {
+  margin: 0 0 20px 0;
+  font-size: 1.25rem;
+  font-weight: 600;
+  color: var(--text-normal, #cdd6f4);
+}
+
+.obsigate-drop-zones {
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+  gap: 16px;
+}
+
+.obsigate-drop-zone {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  padding: 28px 16px;
+  border: 2px dashed var(--border-color, #45475a);
+  border-radius: 8px;
+  background: var(--bg-secondary, #181825);
+  cursor: pointer;
+  transition: all 0.15s ease;
+}
+
+.obsigate-drop-zone.hover {
+  border-color: var(--accent, #89b4fa);
+  background: color-mix(in srgb, var(--accent, #89b4fa) 15%, var(--bg-secondary, #181825));
+  transform: scale(1.02);
+}
+
+.obsigate-drop-zone-icon {
+  font-size: 2.5rem;
+  margin-bottom: 12px;
+}
+
+.obsigate-drop-zone-title {
+  font-size: 1rem;
+  font-weight: 600;
+  color: var(--text-normal, #cdd6f4);
+  margin-bottom: 6px;
+}
+
+.bookslm-composer.bookslm-drop-over {
+  outline: 2px dashed var(--accent, #89b4fa);
+  outline-offset: 2px;
+  background: color-mix(in srgb, var(--accent, #89b4fa) 10%, var(--bg-secondary, #181825));
+}
+
 .tree-children {
   padding-left: 16px;
   overflow: hidden;