fix: 다크모드 UI 개선

- 날짜 이동 팝업 배경을 theme-aware 색상으로 변경

- 토스트 배경 및 글자색 theme-aware로 수정

- 다크모드에서 텍스트 가시성 개선

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

@@ -2,7 +2,7 @@
 <shape xmlns:android="http://schemas.android.com/apk/res/android"
     android:shape="rectangle">
     <!-- 배경색: 다크모드에서도 잘 보이도록 surface 색상 사용 -->
-    <solid android:color="#CC333333" />
+    <solid android:color="@color/surface_variant" />
     <corners android:radius="16dp" />
     <stroke
         android:width="1dp"

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

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <shape xmlns:android="http://schemas.android.com/apk/res/android">
-    <solid android:color="#FFFFFFFF"/> <!-- Fully Opaque to hide everything behind -->
+    <solid android:color="@color/surface"/> <!-- Theme-aware surface color -->
     <corners android:radius="32dp"/>
     <stroke android:width="1.2dp" android:color="#1A000000"/>
 </shape>

app/src/main/res/layout/custom_toast.xml

@@ -13,7 +13,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:textSize="14sp"
-        android:textColor="@android:color/white"
+        android:textColor="@color/text_primary"
         android:maxLines="2"
         android:ellipsize="end" />