fix: address critical review issues in voice overlay

- Clean up voice sessions on WebSocket disconnect (prevents timer leak)
- Guard against missing LLM config in voice_stop send path
- Return overlay to idle on goal_failed (prevents stuck UI)

android/app/src/main/java/com/thisux/droidclaw/connection/CommandRouter.kt

@@ -79,6 +79,7 @@ class CommandRouter(
             }
             "goal_failed" -> {
                 currentGoalStatus.value = GoalStatus.Failed
+                ConnectionService.instance?.overlay?.returnToIdle()
                 Log.i(TAG, "Goal failed: ${msg.message}")
             }