refactor(auth): funnel OAuth signIn into auth state machine

Move new AuthPlatform(provider).signIn() out of UI components and into
the auth state machine. UI now dispatches AuthGoogle/FacebookLoginSubmitted
events; a new oauthSignIn actor inside the machine calls next-auth/react's
signIn(provider). Errors and the OAuth redirect window are exposed via
the existing state.isLoading / state.errorMessage surface, removing the
local busy / error state in auth-facebook-panel.

Co-Authored-By: Claude <noreply@anthropic.com>

src/app/auth/components/auth-facebook-panel.tsx

@@ -7,14 +7,16 @@
  * 视觉规格（与 Dart 对齐）：
  *   - Spacer → logo（120h）→ Spacer → Facebook 主按钮 → 链接 → Spacer → 法律
  *   - Facebook 主按钮：46px 白胶囊 + Facebook "f" 图标（#1877f2）+ "Login with Facebook"
- *   - 社交登录走 NextAuth：`new AuthPlatform("facebook").signIn()` / `new AuthPlatform("google").signIn()`
+ *   - 社交登录统一收口到 Auth 状态机：派发 `AuthFacebookLoginSubmitted` /
+ *     `AuthGoogleLoginSubmitted` 事件，状态机内部调 next-auth/react 的 signIn(provider)
+ *   - busy / error 状态来自 `useAuthState()`（与邮箱登录同源）
  *   - "Other Sign-in Options" 链接打开底部弹层
  *   - 弹层 Email 按钮 → 触发 `onSwitchToEmail`（父级派发 `AuthPanelModeChanged`）
  */
 import Image from "next/image";
 import { useState } from "react";
 
-import { AuthPlatform } from "@/lib/auth/nextauth";
+import { useAuthDispatch, useAuthState } from "@/stores/auth/auth-context";
 
 import { AuthErrorMessage } from "./auth-error-message";
 import { AuthLegalText } from "./auth-legal-text";
@@ -27,31 +29,21 @@ export interface AuthFacebookPanelProps {
 
 export function AuthFacebookPanel({ onSwitchToEmail }: AuthFacebookPanelProps) {
   const [showOptions, setShowOptions] = useState(false);
-  const [busy, setBusy] = useState<null | "facebook" | "google">(null);
-  const [error, setError] = useState<string | null>(null);
+  const { isLoading, errorMessage } = useAuthState();
+  const authDispatch = useAuthDispatch();
 
-  const handleFacebook = async () => {
-    setBusy("facebook");
-    setError(null);
-    try {
-      await new AuthPlatform("facebook").signIn();
-    } catch (e) {
-      setError(e instanceof Error ? e.message : "Facebook login failed");
-    } finally {
-      setBusy(null);
-    }
+  const handleFacebook = () => {
+    authDispatch({
+      type: "AuthFacebookLoginSubmitted",
+      provider: "facebook",
+    });
   };
 
-  const handleGoogle = async () => {
-    setBusy("google");
-    setError(null);
-    try {
-      await new AuthPlatform("google").signIn();
-    } catch (e) {
-      setError(e instanceof Error ? e.message : "Google login failed");
-    } finally {
-      setBusy(null);
-    }
+  const handleGoogle = () => {
+    authDispatch({
+      type: "AuthGoogleLoginSubmitted",
+      provider: "google",
+    });
   };
 
   return (
@@ -70,10 +62,10 @@ export function AuthFacebookPanel({ onSwitchToEmail }: AuthFacebookPanelProps) {
       <button
         type="button"
         className={styles.facebookButton}
-        onClick={() => void handleFacebook()}
-        disabled={busy !== null}
+        onClick={() => handleFacebook()}
+        disabled={isLoading}
       >
-        {busy === "facebook" ? (
+        {isLoading ? (
           <span className={styles.spinner} aria-hidden="true" />
         ) : (
           <span className={styles.facebookIcon} aria-hidden="true">
@@ -83,7 +75,7 @@ export function AuthFacebookPanel({ onSwitchToEmail }: AuthFacebookPanelProps) {
           </span>
         )}
         <span className={styles.facebookLabel}>
-          {busy === "facebook" ? "Logging in..." : "Login with Facebook"}
+          {isLoading ? "Logging in..." : "Login with Facebook"}
         </span>
       </button>
 
@@ -97,15 +89,15 @@ export function AuthFacebookPanel({ onSwitchToEmail }: AuthFacebookPanelProps) {
 
       <div className={styles.spacer} />
 
-      <AuthErrorMessage message={error} />
+      <AuthErrorMessage message={errorMessage} />
       <AuthLegalText />
 
       <AuthOtherOptionsDialog
         open={showOptions}
         onClose={() => setShowOptions(false)}
         mode="facebook"
-        onFacebook={() => void handleFacebook()}
-        onGoogle={() => void handleGoogle()}
+        onFacebook={() => handleFacebook()}
+        onGoogle={() => handleGoogle()}
         onEmail={onSwitchToEmail}
       />
     </div>