fix: BLE 로그 Downloads 폴더에 저장 (파일 탐색기 접근 가능)

- exportToDownloads(): Downloads/VesiScan_BLE_날짜.log 로 저장
- 실패 시 fallback으로 내부 저장소에 저장
- Toast로 저장 경로 안내

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

app/src/main/java/com/example/medilightv2android/services/BleDebugLogger.kt

@@ -207,32 +207,47 @@ class BleDebugLogger private constructor(private val context: Context) {
 
     // ── File Export ──
 
+    private fun writeLogContent(writer: java.io.BufferedWriter) {
+        writer.write("=== VesiScan BLE Debug Log ===\n")
+        writer.write("Device: $deviceName\n")
+        writer.write("Session: ${dateFmt.format(Date(sessionStart))}\n")
+        writer.write("Duration: ${sessionDurationSec()}s\n")
+        writer.write("TX: $txCount  RX: $rxCount  Errors: $errorCount  Timeouts: $packetTimeouts\n")
+
+        if (rssiHistory.isNotEmpty()) {
+            writer.write("RSSI: avg=${rssiHistory.map { it.second }.average().toInt()} ")
+            writer.write("min=${rssiHistory.minOf { it.second }} max=${rssiHistory.maxOf { it.second }}\n")
+        }
+
+        writer.write("=".repeat(60) + "\n\n")
+
+        for (entry in entries) {
+            writer.write(entry.toLogLine() + "\n")
+        }
+    }
+
     fun exportToFile(): File {
         val fileName = "ble_debug_${dateFmt.format(Date())}.log"
         val file = File(logDir, fileName)
-
-        file.bufferedWriter().use { writer ->
-            writer.write("=== VesiScan BLE Debug Log ===\n")
-            writer.write("Device: $deviceName\n")
-            writer.write("Session: ${dateFmt.format(Date(sessionStart))}\n")
-            writer.write("Duration: ${sessionDurationSec()}s\n")
-            writer.write("TX: $txCount  RX: $rxCount  Errors: $errorCount  Timeouts: $packetTimeouts\n")
-
-            if (rssiHistory.isNotEmpty()) {
-                writer.write("RSSI: avg=${rssiHistory.map { it.second }.average().toInt()} ")
-                writer.write("min=${rssiHistory.minOf { it.second }} max=${rssiHistory.maxOf { it.second }}\n")
-            }
-
-            writer.write("=".repeat(60) + "\n\n")
-
-            for (entry in entries) {
-                writer.write(entry.toLogLine() + "\n")
-            }
-        }
-
+        file.bufferedWriter().use { writeLogContent(it) }
         return file
     }
 
+    /** Downloads 폴더에 저장 (파일 탐색기에서 바로 접근 가능) */
+    fun exportToDownloads(): File? {
+        return try {
+            val downloadsDir = android.os.Environment.getExternalStoragePublicDirectory(
+                android.os.Environment.DIRECTORY_DOWNLOADS
+            )
+            val fileName = "VesiScan_BLE_${dateFmt.format(Date())}.log"
+            val file = File(downloadsDir, fileName)
+            file.bufferedWriter().use { writeLogContent(it) }
+            file
+        } catch (e: Exception) {
+            null
+        }
+    }
+
     fun exportToJsonFile(): File {
         val fileName = "ble_debug_${dateFmt.format(Date())}.jsonl"
         val file = File(logDir, fileName)