feat: 다크모드 지원 커스텀 토스트 구현

- showCustomToast() 함수 추가 (AlarmUtils)
- 커스텀 토스트 레이아웃 및 배경 drawable 추가
- values/colors.xml 및 values-night/colors.xml 리소스 사용

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

app/src/main/res/drawable/bg_custom_toast.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android"
+    android:shape="rectangle">
+    <solid android:color="@color/surface" />
+    <corners android:radius="12dp" />
+    <stroke
+        android:width="1dp"
+        android:color="@color/outline" />
+</shape>