fix: download timeout + file validation + source indicator + stale APK

Root cause of "install doesn't update": gradle cached old APK, release
uploaded stale version. Fixes:
- Dedicated download OkHttpClient with 120s readTimeout (was 30s)
- 64KB buffer (was 8KB) for faster large-file download
- Post-download file size validation (Content-Length / release declared)
- Reject files < 1MB (likely HTML error pages)
- downloadSource StateFlow shows "Gitea" or "GitHub" during download
- UpdateDialog shows "正在下载更新…（来自 Gitea）"
- version 1.9.2 -> 1.9.3 (code 31 -> 32)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/java/com/a2i/forwarder/core/UpdateChecker.kt

@@ -45,6 +45,7 @@ class UpdateChecker(private val context: Context) {
     val updateInfo = MutableStateFlow<UpdateInfo?>(null)
     val downloading = MutableStateFlow(false)
     val downloadProgress = MutableStateFlow(0)
+    val downloadSource = MutableStateFlow<String?>(null)  // "Gitea" / "GitHub" / null
     val lastError = MutableStateFlow<String?>(null)
     private var lastNotifiedVersion: String? = null  // 记录已推送的版本，避免重复通知
 
@@ -184,33 +185,49 @@ class UpdateChecker(private val context: Context) {
     suspend fun download(info: UpdateInfo): File? = withContext(Dispatchers.IO) {
         downloading.value = true
         downloadProgress.value = 0
+        downloadSource.value = null
         lastError.value = null
-        // 主 URL 失败时自动尝试备选 URL
-        val urls = listOf(info.apkUrl) + (info.fallbackUrl?.let { listOf(it) } ?: emptyList())
-        for ((idx, url) in urls.withIndex()) {
-            if (idx > 0) downloadProgress.value = 0  // 重试时重置进度
-            val file = tryDownload(url, info.latestVersion)
+        // 主 URL (Gitea) 失败时自动尝试备选 URL (GitHub)
+        val sources = listOf(
+            info.apkUrl to sourceLabel(info.apkUrl),
+        ) + (info.fallbackUrl?.let { listOf(it to sourceLabel(it)) } ?: emptyList())
+        for ((idx, pair) in sources.withIndex()) {
+            val (url, label) = pair
+            if (idx > 0) { downloadProgress.value = 0; downloadSource.value = null }
+            downloadSource.value = label
+            val file = tryDownload(url, info.latestVersion, info.apkSize)
             if (file != null) {
                 downloading.value = false
+                downloadSource.value = null
                 return@withContext file
             }
         }
         downloading.value = false
+        downloadSource.value = null
         null
     }
 
-    private suspend fun tryDownload(url: String, version: String): File? {
+    private fun sourceLabel(url: String): String =
+        if (url.contains("songer.eu.org") || url.contains("sunlunfan")) "Gitea" else "GitHub"
+
+    private suspend fun tryDownload(url: String, version: String, expectedSize: Long): File? {
+        // 专用下载 client：更长超时（120s），避免大文件慢网超时截断
+        val dlClient = OkHttpClient.Builder()
+            .connectTimeout(15, TimeUnit.SECONDS)
+            .readTimeout(120, TimeUnit.SECONDS)
+            .retryOnConnectionFailure(true)
+            .build()
         return try {
             val req = Request.Builder().url(url).get().build()
-            http.newCall(req).execute().use { res ->
-                if (!res.isSuccessful) { lastError.value = "HTTP ${res.code}"; return@tryDownload null }
-                val body = res.body ?: run { lastError.value = "空响应"; return@tryDownload null }
+            dlClient.newCall(req).execute().use { res ->
+                if (!res.isSuccessful) { lastError.value = "HTTP ${res.code}"; return null }
+                val body = res.body ?: run { lastError.value = "空响应"; return null }
                 val total = body.contentLength()
                 val dir = context.getExternalFilesDir(null) ?: context.cacheDir
                 val target = File(dir, "gotmsg-${version}.apk")
                 target.outputStream().use { sink ->
                     val input = body.byteStream()
-                    val buf = ByteArray(8 * 1024)
+                    val buf = ByteArray(64 * 1024)  // 64KB buffer 提升大文件速度
                     var read = 0L
                     while (true) {
                         val n = input.read(buf)
@@ -220,6 +237,19 @@ class UpdateChecker(private val context: Context) {
                         if (total > 0) downloadProgress.value = (read * 100 / total).toInt()
                     }
                 }
+                // 校验文件大小：Content-Length 或 release 声明的大小
+                val actualSize = target.length()
+                val refSize = if (total > 0) total else expectedSize
+                if (refSize > 0 && actualSize < refSize) {
+                    lastError.value = "文件不完整（${actualSize}/${refSize} 字节），请重试"
+                    target.delete()
+                    return null
+                }
+                if (actualSize < 1_000_000) {
+                    lastError.value = "文件过小，可能下载了错误页面"
+                    target.delete()
+                    return null
+                }
                 downloadProgress.value = 100
                 target
             }

app/src/main/java/com/a2i/forwarder/ui/screens/UpdateDialog.kt

@@ -37,6 +37,7 @@ fun UpdateDialog(info: UpdateInfo, onDismiss: () -> Unit) {
     val app = A2iApp.instance
     val downloading by app.updateChecker.downloading.collectAsState()
     val progress by app.updateChecker.downloadProgress.collectAsState()
+    val downloadSource by app.updateChecker.downloadSource.collectAsState()
     var downloadedFile by remember { mutableStateOf<File?>(null) }
 
     AlertDialog(
@@ -70,7 +71,7 @@ fun UpdateDialog(info: UpdateInfo, onDismiss: () -> Unit) {
                     )
                     Spacer(Modifier.size(4.dp))
                     Text(
-                        "正在下载更新…",
+                        "正在下载更新…${downloadSource?.let { "（来自 $it）" } ?: ""}",
                         style = MaterialTheme.typography.bodySmall,
                         color = MaterialTheme.colorScheme.onSurfaceVariant,
                     )