Add OAuth 2.0 authorization flow documentation and troubleshooting guide

- Created AUTH/OAUTH_FLOW.MD to detail the OAuth 2.0 authorization process for SECTL Auth, including flow diagrams, detailed steps, token management, and security recommendations.
- Added AUTH/TROUBLESHOOTING.md to provide a comprehensive troubleshooting guide for common issues encountered during the OAuth process, covering authorization, token exchange, user info retrieval, remote logout, platform management, and network/CORS issues.
- Implemented OAuthCallback and OAuthLogin components in React to handle OAuth login flow, including deep linking and token exchange.

src/components/OAuthCallback.tsx

@@ -0,0 +1,49 @@
+import { useEffect } from "react"
+import { useSearchParams, useNavigate } from "react-router-dom"
+
+export function OAuthCallback() {
+  const [searchParams] = useSearchParams()
+  const navigate = useNavigate()
+
+  useEffect(() => {
+    const code = searchParams.get("code")
+    const error = searchParams.get("error")
+    const errorDescription = searchParams.get("error_description")
+
+    if (error) {
+      window.postMessage(
+        {
+          error: errorDescription || error,
+        },
+        window.location.origin
+      )
+      navigate("/")
+      return
+    }
+
+    if (code) {
+      window.postMessage(
+        {
+          code,
+        },
+        window.location.origin
+      )
+      navigate("/")
+    }
+  }, [searchParams, navigate])
+
+  return (
+    <div
+      style={{
+        display: "flex",
+        justifyContent: "center",
+        alignItems: "center",
+        height: "100vh",
+        background: "var(--ss-bg-color)",
+        color: "var(--ss-text-main)",
+      }}
+    >
+      <div>正在处理登录...</div>
+    </div>
+  )
+}