Fix compile error: move continue outside withContext lambda

Return null from withContext on network error and continue at the
outer loop level, since Kotlin disallows break/continue across
function boundaries.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: xiaocang

android/app/src/main/kotlin/com/ghpr/app/auth/GitHubOAuthManager.kt

@@ -170,7 +170,7 @@ class GitHubOAuthManager(context: Context) {
                 .post(body)
                 .build()
 
-            val payload = withContext(Dispatchers.IO) {
+            val payload: String? = withContext(Dispatchers.IO) {
                 try {
                     client.newCall(request).execute().use { response ->
                         val raw = response.body?.string().orEmpty()
@@ -188,10 +188,12 @@ class GitHubOAuthManager(context: Context) {
                             pollingError = e.message ?: "Network error",
                         )
                     }
-                    continue
+                    null
                 }
             }
 
+            if (payload == null) continue
+
             // Clear polling error on successful response
             val current = _authState.value
             if (current is GitHubAuthState.Authenticating && current.pollingError != null) {