fix(webui): preserve session drag contracts

webui/src/tests/chat-list.test.tsx

@@ -88,6 +88,8 @@ describe("ChatList", () => {
       session({ chatId: "alpha", title: "Alpha" }),
       session({ chatId: "bravo", title: "Bravo" }),
       session({ chatId: "charlie", title: "Charlie" }),
+      session({ chatId: "old-a", title: "Old A" }),
+      session({ chatId: "old-b", title: "Old B" }),
     ];
     const { rerender } = render(
       <ChatList
@@ -99,6 +101,8 @@ describe("ChatList", () => {
         onRequestRename={vi.fn()}
         onToggleArchive={vi.fn()}
         onReorderSessions={onReorderSessions}
+        archivedKeys={["websocket:old-a", "websocket:old-b"]}
+        sessionOrder={sessions.map((item) => item.key)}
       />,
     );
     const dataTransfer = {
@@ -117,6 +121,8 @@ describe("ChatList", () => {
       "websocket:bravo",
       "websocket:charlie",
       "websocket:alpha",
+      "websocket:old-a",
+      "websocket:old-b",
     ]);
 
     rerender(
@@ -129,10 +135,13 @@ describe("ChatList", () => {
         onRequestRename={vi.fn()}
         onToggleArchive={vi.fn()}
         onReorderSessions={onReorderSessions}
+        archivedKeys={["websocket:old-a", "websocket:old-b"]}
         sessionOrder={[
           "websocket:bravo",
           "websocket:charlie",
           "websocket:alpha",
+          "websocket:old-a",
+          "websocket:old-b",
         ]}
         sort="manual"
       />,

webui/src/tests/thread-composer.test.tsx

@@ -1647,6 +1647,28 @@ describe("ThreadComposer", () => {
     });
   });
 
+  it("rejects session drops that are unavailable to the composer", () => {
+    render(
+      <ThreadComposer
+        onSend={vi.fn()}
+        placeholder="Type your message..."
+        sessions={[]}
+      />,
+    );
+    const input = screen.getByLabelText("Message input");
+    const dataTransfer = {
+      types: [SESSION_DRAG_TYPE],
+      effectAllowed: "copyMove",
+      dropEffect: "copy",
+      files: [],
+      getData: () => "websocket:current",
+    };
+
+    expect(fireEvent.dragEnter(input, { dataTransfer })).toBe(true);
+    expect(fireEvent.dragOver(input, { dataTransfer })).toBe(true);
+    expect(screen.queryByTestId("composer-session-drag-preview")).not.toBeInTheDocument();
+  });
+
   it("disambiguates duplicate and capability-colliding session names", () => {
     render(
       <ThreadComposer