fix(auth): include OAuth backend-sync phase in isLoading derivation

The AuthState.isLoading derivation in auth-context.tsx covered the
OAuth redirect phase (loadingOAuth) but NOT the backend-sync phase
that runs after the OAuth callback returns
(syncingGoogleBackend / syncingFacebookBackend). Any UI that
read isLoading would incorrectly report "not loading" during
the backend token-exchange round-trip.

Add the two missing state.matches() lines so isLoading is true
for the entire OAuth login window (button click → OAuth redirect
→ callback → backend sync).

Note: in the current OAuth flow the user is on /chat during the
sync phase (NextAuth redirected them there), so the splash/auth
button is not actually visible to reflect the new true value.
The fix is still correct because:

- Conceptually aligned: any in-flight login op is "loading"
- Defense in depth: future UI on /chat that consumes isLoading
  will get accurate feedback
- Non-regression: only makes isLoading more permissive (true more
  often), never false-fires

A visible /chat overlay during the sync phase would be a separate,
larger change — noted in the planning doc but not in this commit.

src/stores/auth/auth-context.tsx

@@ -61,11 +61,14 @@ export function AuthProvider({ children }: AuthProviderProps) {
       password: state.context.password,
       username: state.context.username,
       confirmPassword: state.context.confirmPassword,
-      // isLoading 覆盖邮箱登录 / 邮箱注册 / OAuth 跳转（NextAuth 重定向期间）/ 显式游客登录
+      // isLoading 覆盖邮箱登录 / 邮箱注册 / OAuth 跳转（NextAuth 重定向期间）/
+      // OAuth 回调后端 sync / 显式游客登录
       isLoading:
         state.matches("loadingEmailLogin") ||
         state.matches("loadingEmailRegister") ||
         state.matches("loadingOAuth") ||
+        state.matches("syncingGoogleBackend") ||
+        state.matches("syncingFacebookBackend") ||
         state.matches("loadingGuestLogin"),
       errorMessage: state.context.errorMessage,
       loginStatus: state.context.loginStatus,