Make chip selection background green to match app theme

Add custom chip_duration_bg selector drawable (green when checked,
light gray when unchecked) and wire it through a ChipDuration chipStyle
in the theme. This overrides Material3's default purple checked state.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<selector xmlns:android="http://schemas.android.com/apk/res/android">
+    <item android:state_checked="true">
+        <shape android:shape="rectangle">
+            <corners android:radius="12dp" />
+            <solid android:color="#81C784" />
+        </shape>
+    </item>
+    <item>
+        <shape android:shape="rectangle">
+            <corners android:radius="12dp" />
+            <solid android:color="#F5F5F5" />
+        </shape>
+    </item>
+</selector>

app/src/main/res/values/themes.xml

@@ -1,12 +1,19 @@
 <resources>
     <style name="Theme.CountdownTimer" parent="Theme.Material3.DayNight.NoActionBar">
         <item name="colorPrimary">@color/md_theme_light_primary</item>
+        <item name="colorPrimaryContainer">@color/md_theme_light_primaryContainer</item>
         <item name="colorSecondary">@color/md_theme_light_secondary</item>
         <item name="android:statusBarColor">@color/color_gradient_start</item>
         <item name="android:windowLightStatusBar">true</item>
+        <item name="chipStyle">@style/ChipDuration</item>
+    </style>
+
+    <style name="ChipDuration" parent="Widget.Material3.Chip.Suggestion.Elevated">
+        <item name="chipBackground">@drawable/chip_duration_bg</item>
     </style>
 
     <color name="md_theme_light_primary">#2E7D32</color>
+    <color name="md_theme_light_primaryContainer">#81C784</color>
     <color name="md_theme_light_secondary">#4C6842</color>
     <color name="md_theme_light_surface">#F1F8E9</color>
 </resources>