fix(auth): surface validation errors in email login/register forms

Validation failure was previously silent — the form's submit() returned
without dispatching or showing anything on screen, so users could not tell
why the login/register button appeared to do nothing.

Changes:
- Add validationError local state in EmailLoginForm and EmailRegisterForm
- Display first validation error via AuthErrorMessage, prioritised over
  the server's globalError (validation errors feel more immediate)
- Clear validation error on field change (standard "error fades as you
  fix it" UX) — implemented as onChange handlers, not useEffect, to
  avoid React's cascading-render anti-pattern
- Add observability to the entire email flow (was previously silent end
  to end): form submit ENTRY, validator rejection, actor ENTRY, actor
  DONE — mirrors the logging style already used by guestLoginActor
- Add entry: assign({ errorMessage: null }) to loadingEmailLogin and
  loadingEmailRegister so stale errors clear on transition into the
  loading state (matches loadingGuestLogin)

src/stores/auth/auth-machine.ts

@@ -83,21 +83,21 @@ export const authMachine = setup({
         },
         // 启动 / 恢复时检查登录态 —— 由 <AuthStatusChecker /> 派发
         AuthStatusCheckSubmitted: "checkingAuthStatus",
-        // 显式游客登录 —— 由 splash UI 派发（不自动）
+
         AuthGuestLoginSubmitted: "loadingGuestLogin",
         AuthEmailLoginSubmitted: "loadingEmailLogin",
         AuthEmailRegisterSubmitted: "loadingEmailRegister",
         AuthGoogleLoginSubmitted: "loadingOAuth",
         AuthFacebookLoginSubmitted: "loadingOAuth",
-        
-        // OAuth 回调后 sync 入口 —— 由 <OAuthSessionSync /> 派发
-        AuthGoogleSyncSubmitted: "syncingGoogleBackend",
-        AuthFacebookSyncSubmitted: "syncingFacebookBackend",
         AuthAppleLoginSubmitted: {
           actions: assign({
             errorMessage: "Apple login not implemented",
           }),
         },
+        
+        // OAuth 回调后 sync 入口 —— 由 <OAuthSessionSync /> 派发
+        AuthGoogleSyncSubmitted: "syncingGoogleBackend",
+        AuthFacebookSyncSubmitted: "syncingFacebookBackend",
         // splash / auth screen 跳完 /chat 后清flag
         AuthClearPendingRedirect: {
           actions: assign({ pendingRedirect: false }),
@@ -151,6 +151,7 @@ export const authMachine = setup({
     },
 
     loadingEmailLogin: {
+      entry: assign({ errorMessage: null }),
       invoke: {
         src: "emailLogin",
         input: ({ event }) => {
@@ -177,6 +178,7 @@ export const authMachine = setup({
     },
 
     loadingEmailRegister: {
+      entry: assign({ errorMessage: null }),
       invoke: {
         src: "emailRegisterThenLogin",
         input: ({ event }) => {