fix(webui): default to new chat on load and preserve scroll on settings return

- Remove auto-selection of the most recent session on initial load,
  so the app opens to a blank new-chat page instead of the last session.
- Preserve active session state when navigating to/from settings:
  keep ThreadShell mounted (hidden via CSS) so scroll position, message
  cache, and streaming state are not lost.
- Update onBackToChat to return to blank page when no session was active
  instead of falling back to the most recent session.
- Update related test expectations to match the new navigation behavior.

webui/src/App.tsx

@@ -250,7 +250,6 @@ function Shell({ onModelNameChange, onLogout }: { onModelNameChange: (modelName:
     key: string;
     label: string;
   } | null>(null);
-  const lastSessionsLen = useRef(0);
   const restartSawDisconnectRef = useRef(false);
   const [restartToast, setRestartToast] = useState<string | null>(null);
   const [isRestarting, setIsRestarting] = useState(false);
@@ -266,13 +265,7 @@ function Shell({ onModelNameChange, onLogout }: { onModelNameChange: (modelName:
     }
   }, [desktopSidebarOpen]);
 
-  useEffect(() => {
-    if (activeKey) return;
-    if (sessions.length > 0 && lastSessionsLen.current === 0) {
-      setActiveKey(sessions[0].key);
-    }
-    lastSessionsLen.current = sessions.length;
-  }, [sessions, activeKey]);
+
 
   const activeSession = useMemo<ChatSummary | null>(() => {
     if (!activeKey) return null;
@@ -335,9 +328,8 @@ function Shell({ onModelNameChange, onLogout }: { onModelNameChange: (modelName:
     setView("chat");
     setMobileSidebarOpen(false);
     setActiveKey((current) => {
-      if (current && sessions.some((session) => session.key === current)) {
-        return current;
-      }
+      if (!current) return null;
+      if (sessions.some((session) => session.key === current)) return current;
       return sessions[0]?.key ?? null;
     });
   }, [sessions]);
@@ -479,18 +471,13 @@ function Shell({ onModelNameChange, onLogout }: { onModelNameChange: (modelName:
         </Sheet>
       ) : null}
 
-      <main className="flex h-full min-w-0 flex-1 flex-col">
-        {view === "settings" ? (
-          <SettingsView
-            theme={theme}
-            onToggleTheme={toggle}
-            onBackToChat={onBackToChat}
-            onModelNameChange={onModelNameChange}
-            onLogout={onLogout}
-            onRestart={onRestart}
-            isRestarting={isRestarting}
-          />
-        ) : (
+      <main className="relative flex h-full min-w-0 flex-1 flex-col">
+        <div
+          className={cn(
+            "absolute inset-0 flex flex-col",
+            view === "settings" && "invisible pointer-events-none",
+          )}
+        >
           <ThreadShell
             session={activeSession}
             title={headerTitle}
@@ -502,6 +489,19 @@ function Shell({ onModelNameChange, onLogout }: { onModelNameChange: (modelName:
             onToggleTheme={toggle}
             hideSidebarToggleOnDesktop={desktopSidebarOpen}
           />
+        </div>
+        {view === "settings" && (
+          <div className="absolute inset-0 flex flex-col">
+            <SettingsView
+              theme={theme}
+              onToggleTheme={toggle}
+              onBackToChat={onBackToChat}
+              onModelNameChange={onModelNameChange}
+              onLogout={onLogout}
+              onRestart={onRestart}
+              isRestarting={isRestarting}
+            />
+          </div>
         )}
       </main>