fix: watchdog을 별도 Thread로 분리 (handler 블로킹 방지)

기존: handler.postDelayed로 watchdog 실행 → BLE 스택이 handler를
블로킹하면 watchdog도 멈춤 (좀비 상태 감지 불가)

변경: 별도 daemon Thread에서 5초마다 lastRxTimestamp 체크
→ BLE handler가 블로킹되어도 독립적으로 15초 타임아웃 감지
→ 감지 시 handler.post로 메인 스레드에서 forceDisconnectAndReconnect 실행

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

app/src/main/java/com/example/medilightv2android/ble/BleManager.kt

@@ -352,31 +352,39 @@ class BleManager private constructor(private val context: Context) {
         batteryTimer = null
     }
 
-    // ── Watchdog (zombie connection detection) ──
+    // ── Watchdog (zombie connection detection, runs on separate thread) ──
+
+    private var watchdogJob: Thread? = null
+    @Volatile private var watchdogRunning = false
 
     private fun startWatchdog() {
         stopWatchdog()
         lastRxTimestamp = System.currentTimeMillis()
-        val check = object : Runnable {
-            override fun run() {
-                if (!isConnected.value) return
+        watchdogRunning = true
+
+        watchdogJob = Thread({
+            while (watchdogRunning) {
+                try { Thread.sleep(5000) } catch (_: InterruptedException) { break }
+                if (!watchdogRunning) break
+                if (!isConnected.value) continue
+
                 val silenceMs = System.currentTimeMillis() - lastRxTimestamp
                 if (silenceMs > watchdogTimeoutMs) {
+                    handler.post {
                         logw { "Watchdog: no RX for ${silenceMs}ms — forcing disconnect + reconnect" }
                         debugLogger.error("WATCHDOG_TIMEOUT silence=${silenceMs}ms — forcing reconnect")
                         forceDisconnectAndReconnect()
-                    return
                     }
-                handler.postDelayed(this, 5000)
+                    break
                 }
             }
-        watchdogTimer = check
-        handler.postDelayed(check, 5000)
+        }, "BLE-Watchdog").also { it.isDaemon = true; it.start() }
     }
 
     private fun stopWatchdog() {
-        watchdogTimer?.let { handler.removeCallbacks(it) }
-        watchdogTimer = null
+        watchdogRunning = false
+        watchdogJob?.interrupt()
+        watchdogJob = null
     }
 
     private fun forceDisconnectAndReconnect() {