feat(left-bar): add drop zone for ungrouping when all sessions are in groups

- Shows "Drop here to ungroup" zone when dragging a session and no ungrouped sessions exist
- Allows users to drag sessions out of groups even when Ungrouped Agents folder is hidden
- Added tests for drop zone visibility and drop handler

src/__tests__/renderer/components/SessionList.test.tsx

@@ -1195,6 +1195,45 @@ describe('SessionList', () => {
 
 			expect(handleDropOnGroup).toHaveBeenCalledWith('g1');
 		});
+
+		it('shows drop zone for ungrouping when dragging and all sessions are grouped', () => {
+			const handleDropOnUngrouped = vi.fn();
+			const group = createMockGroup({ id: 'g1', name: 'My Group', sessionIds: ['s1'] });
+			const sessions = [createMockSession({ id: 's1', name: 'Grouped Session', groupId: 'g1' })];
+			const props = createDefaultProps({
+				sessions,
+				sortedSessions: sessions,
+				groups: [group],
+				leftSidebarOpen: true,
+				draggingSessionId: 's1', // Simulating active drag
+				handleDropOnUngrouped,
+			});
+			render(<SessionList {...props} />);
+
+			// Drop zone should be visible when dragging
+			expect(screen.getByText('Drop here to ungroup')).toBeInTheDocument();
+		});
+
+		it('calls handleDropOnUngrouped when dropping on ungroup zone', () => {
+			const handleDropOnUngrouped = vi.fn();
+			const group = createMockGroup({ id: 'g1', name: 'My Group', sessionIds: ['s1'] });
+			const sessions = [createMockSession({ id: 's1', name: 'Grouped Session', groupId: 'g1' })];
+			const props = createDefaultProps({
+				sessions,
+				sortedSessions: sessions,
+				groups: [group],
+				leftSidebarOpen: true,
+				draggingSessionId: 's1',
+				handleDropOnUngrouped,
+			});
+			render(<SessionList {...props} />);
+
+			// Find the drop zone and drop on it
+			const dropZone = screen.getByText('Drop here to ungroup');
+			fireEvent.drop(dropZone);
+
+			expect(handleDropOnUngrouped).toHaveBeenCalled();
+		});
 	});
 
 	// ============================================================================

src/renderer/components/SessionList.tsx

@@ -2726,8 +2726,25 @@ function SessionListInner(props: SessionListProps) {
 							)}
 						</div>
 					) : groups.length > 0 ? (
-						/* NO UNGROUPED AGENTS - Show standalone New Group button */
-						<div className="mt-4 px-3">
+						/* NO UNGROUPED AGENTS - Show drop zone for ungrouping + New Group button */
+						<div
+							className="mt-4 px-3"
+							onDragOver={handleDragOver}
+							onDrop={handleDropOnUngrouped}
+						>
+							{/* Drop zone indicator when dragging */}
+							{draggingSessionId && (
+								<div
+									className="mb-2 px-3 py-2 rounded border-2 border-dashed text-center text-xs"
+									style={{
+										borderColor: theme.colors.accent,
+										color: theme.colors.textDim,
+										backgroundColor: theme.colors.accent + '10',
+									}}
+								>
+									Drop here to ungroup
+								</div>
+							)}
 							<button
 								onClick={createNewGroup}
 								className="w-full px-2 py-1.5 rounded-full text-[10px] font-medium hover:opacity-80 transition-opacity flex items-center justify-center gap-1"