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/layout/custom_toast.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="utf-8"?>
+<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="wrap_content"
+    android:layout_height="wrap_content"
+    android:orientation="horizontal"
+    android:background="@drawable/bg_custom_toast"
+    android:paddingHorizontal="16dp"
+    android:paddingVertical="10dp"
+    android:gravity="center">
+
+    <TextView
+        android:id="@+id/toastText"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:textSize="14sp"
+        android:textColor="@color/text_primary"
+        android:maxLines="2"
+        android:ellipsize="end" />
+
+</LinearLayout>