feat: multi-account switcher in the avatar menu

SessionManager already stores one session per uid; this adds the UI and
the two accessors it was missing. The avatar menu now lists every stored
session — active entries switch in place (page reload re-derives all
state), signed-out entries keep their entry and route to the login page
with the email prefilled — plus an always-visible Add account action.

- session: listSessions() and switchTo(uid) accessors
- UserAction: account section under a divider with per-session avatar,
  nickname/email, signed-out badge, and Add account
- locales: navbar.addAccount / navbar.signedOut (en-US, zh-CN)

Authored By: TDvorak <info@tdvorak.dev>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

frontend/public/locales/en-US/application.json

@@ -163,7 +163,9 @@
     "toDarkMode": "Dark",
     "toLightMode": "Light",
     "myProfile": "My profile",
-    "dashboard": "Dashboard"
+    "dashboard": "Dashboard",
+    "addAccount": "Add account",
+    "signedOut": "Signed out"
   },
   "fileManager": {
     "continueInDesktop": "Authorization successful, please continue in the desktop client.",

frontend/public/locales/zh-CN/application.json

@@ -163,7 +163,9 @@
     "toDarkMode": "黑暗",
     "toLightMode": "浅色",
     "myProfile": "个人主页",
-    "dashboard": "管理面板"
+    "dashboard": "管理面板",
+    "addAccount": "添加账号",
+    "signedOut": "已退出登录"
   },
   "fileManager": {
     "continueInDesktop": "授权成功，请在桌面客户端中继续操作。",

frontend/src/component/Frame/NavBar/UserAction.tsx

@@ -24,6 +24,7 @@ import SessionManager, { Session } from "../../../session";
 import { GroupBS } from "../../../session/utils.ts";
 import UserAvatar from "../../Common/User/UserAvatar.tsx";
 import { SquareMenuItem } from "../../FileManager/ContextMenu/ContextMenu.tsx";
+import Add from "../../Icons/Add.tsx";
 import HomeOutlined from "../../Icons/HomeOutlined.tsx";
 import Person from "../../Icons/Person.tsx";
 import SettingsOutlined from "../../Icons/SettingsOutlined.tsx";
@@ -59,6 +60,27 @@ const UserPopover = ({ open, onClose, ...rest }: PopoverProps) => {
     onClose && onClose({}, "backdropClick");
   }, []);
 
+  // Other stored sessions for the account switcher. Signed-out entries
+  // route back to the login page with their email prefilled instead of
+  // switching directly.
+  const otherSessions = SessionManager.listSessions().filter((s) => s.user.id !== user.id);
+
+  const switchAccount = (s: Session) => {
+    onClose?.({}, "backdropClick");
+    if (s.signedOut) {
+      signWithHint(s.user.email ?? "");
+      return;
+    }
+    if (SessionManager.switchTo(s.user.id)) {
+      window.location.reload();
+    }
+  };
+
+  const addAccount = () => {
+    navigate("/session");
+    onClose?.({}, "backdropClick");
+  };
+
   const openMyProfile = useCallback(() => {
     navigate(`/profile/${user?.id}`);
     onClose && onClose({}, "backdropClick");
@@ -143,6 +165,26 @@ const UserPopover = ({ open, onClose, ...rest }: PopoverProps) => {
           <ListItemText>{t("login.logout")}</ListItemText>
         </SquareMenuItem>
       </MenuList>
+      <Divider />
+      <MenuList dense sx={{ mx: 0.5 }}>
+        {otherSessions.map((s) => (
+          <SquareMenuItem key={s.user.id} onClick={() => switchAccount(s)}>
+            <ListItemIcon>
+              <UserAvatar sx={{ width: 24, height: 24 }} user={s.user} />
+            </ListItemIcon>
+            <ListItemText
+              primary={s.user.nickname}
+              secondary={s.signedOut ? t("navbar.signedOut") : s.user.email}
+            />
+          </SquareMenuItem>
+        ))}
+        <SquareMenuItem onClick={addAccount}>
+          <ListItemIcon>
+            <Add fontSize={"small"} />
+          </ListItemIcon>
+          <ListItemText>{t("navbar.addAccount")}</ListItemText>
+        </SquareMenuItem>
+      </MenuList>
     </Popover>
   );
 };

frontend/src/session/index.ts

@@ -271,6 +271,24 @@ export class Manager {
     this.syncToStore();
   }
 
+  // listSessions returns every stored session for the account switcher.
+  // Signed-out entries are kept so the UI can offer one-click re-login.
+  public listSessions = (): Session[] => {
+    return Object.values(this.state.sessions);
+  };
+
+  // switchTo makes another stored session current; callers reload the app
+  // so all state re-derives from the new session.
+  public switchTo = (uid: string) => {
+    const session = this.state.sessions[uid];
+    if (!session || session.signedOut) {
+      return false;
+    }
+    this.state.current = uid;
+    this.syncToStore();
+    return true;
+  };
+
   private syncToStore = () => {
     localStorage.setItem(SESSION_KEY, JSON.stringify(this.state));
   };