Chat UI redesign, splash screen, and onboarding carousel

- Redesign chat screens (both apps) to match Figma: pink theme with
  doodle pattern background, white app bar with centered name and
  chevron back, rose sender bubbles, white receiver bubbles, entry
  banners, and session-ended bottom bar
- Add splash_chat_hebat.png as native Android splash screen with
  Android 12+ support (values-v31)
- Add Flutter splash screen using splash_chat_hebat.png
- Add onboarding carousel (client_app only): 3 pages with 1s
  auto-advance, last page manual "Mulai" button, first-launch only
- Register image assets in both pubspec.yaml files

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mitra_app/android/app/src/main/res/drawable-v21/launch_background.xml

@@ -1,12 +1,12 @@
 <?xml version="1.0" encoding="utf-8"?>
-<!-- Modify this file to customize your launch splash screen -->
 <layer-list xmlns:android="http://schemas.android.com/apk/res/android">
-    <item android:drawable="?android:colorBackground" />
-
-    <!-- You can insert your own image assets here -->
-    <!-- <item>
+    <item android:drawable="@android:color/white" />
+    <item
+        android:width="200dp"
+        android:height="168dp"
+        android:gravity="center">
         <bitmap
-            android:gravity="center"
-            android:src="@mipmap/launch_image" />
-    </item> -->
+            android:gravity="fill"
+            android:src="@drawable/splash_chat_hebat" />
+    </item>
 </layer-list>

mitra_app/android/app/src/main/res/drawable/launch_background.xml

@@ -1,12 +1,12 @@
 <?xml version="1.0" encoding="utf-8"?>
-<!-- Modify this file to customize your launch splash screen -->
 <layer-list xmlns:android="http://schemas.android.com/apk/res/android">
     <item android:drawable="@android:color/white" />
-
-    <!-- You can insert your own image assets here -->
-    <!-- <item>
+    <item
+        android:width="200dp"
+        android:height="168dp"
+        android:gravity="center">
         <bitmap
-            android:gravity="center"
-            android:src="@mipmap/launch_image" />
-    </item> -->
+            android:gravity="fill"
+            android:src="@drawable/splash_chat_hebat" />
+    </item>
 </layer-list>

mitra_app/android/app/src/main/res/values-v31/styles.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <style name="LaunchTheme" parent="@android:style/Theme.Light.NoTitleBar">
+        <item name="android:windowSplashScreenBackground">@android:color/white</item>
+        <item name="android:windowSplashScreenAnimatedIcon">@drawable/splash_chat_hebat</item>
+    </style>
+    <style name="NormalTheme" parent="@android:style/Theme.Light.NoTitleBar">
+        <item name="android:windowBackground">?android:colorBackground</item>
+    </style>
+</resources>