fix(frontend/copilot): hide New Chat button on Autopilot homepage (#12321)

Requested by @0ubbe

The **New Chat** button was visible on the Autopilot homepage where
clicking it has no effect (since `sessionId` is already `null`). This
hides the button when no chat session is active, so it only appears when
the user is viewing a conversation and wants to start a new one.

**Changes:**
- `ChatSidebar.tsx` — hide button in both collapsed and expanded sidebar
states when `sessionId` is null
- `MobileDrawer.tsx` — same fix for mobile drawer

---
Co-authored-by: Ubbe <ubbe@users.noreply.github.com>

autogpt_platform/frontend/src/app/(platform)/copilot/components/ChatSidebar/ChatSidebar.tsx

@@ -202,14 +202,16 @@ export function ChatSidebar() {
             >
               <div className="flex flex-col items-center gap-2">
                 <SidebarTrigger />
-                <Button
-                  variant="ghost"
-                  onClick={handleNewChat}
-                  style={{ minWidth: "auto", width: "auto" }}
-                >
-                  <PlusCircleIcon className="!size-5" />
-                  <span className="sr-only">New Chat</span>
-                </Button>
+                {sessionId ? (
+                  <Button
+                    variant="ghost"
+                    onClick={handleNewChat}
+                    style={{ minWidth: "auto", width: "auto" }}
+                  >
+                    <PlusCircleIcon className="!size-5" />
+                    <span className="sr-only">New Chat</span>
+                  </Button>
+                ) : null}
               </div>
             </motion.div>
           </SidebarHeader>
@@ -230,15 +232,17 @@ export function ChatSidebar() {
                   <SidebarTrigger />
                 </div>
               </div>
-              <Button
-                variant="primary"
-                size="small"
-                onClick={handleNewChat}
-                className="w-full"
-                leftIcon={<PlusIcon className="h-4 w-4" weight="bold" />}
-              >
-                New Chat
-              </Button>
+              {sessionId ? (
+                <Button
+                  variant="primary"
+                  size="small"
+                  onClick={handleNewChat}
+                  className="w-full"
+                  leftIcon={<PlusIcon className="h-4 w-4" weight="bold" />}
+                >
+                  New Chat
+                </Button>
+              ) : null}
             </motion.div>
           </SidebarHeader>
         )}

autogpt_platform/frontend/src/app/(platform)/copilot/components/MobileDrawer/MobileDrawer.tsx

@@ -71,17 +71,19 @@ export function MobileDrawer({
                 <X width="1rem" height="1rem" />
               </Button>
             </div>
-            <div className="mt-2">
-              <Button
-                variant="primary"
-                size="small"
-                onClick={onNewChat}
-                className="w-full"
-                leftIcon={<PlusIcon width="1rem" height="1rem" />}
-              >
-                New Chat
-              </Button>
-            </div>
+            {currentSessionId ? (
+              <div className="mt-2">
+                <Button
+                  variant="primary"
+                  size="small"
+                  onClick={onNewChat}
+                  className="w-full"
+                  leftIcon={<PlusIcon width="1rem" height="1rem" />}
+                >
+                  New Chat
+                </Button>
+              </div>
+            ) : null}
           </div>
           <div
             className={cn(