feat(splash): implement unified splash screen for all platforms

- Add splash image to Android launch background
- Update pubspec.yaml to include splash assets
- Modify web index.html for consistent splash behavior
- Remove unused loading indicator and spinner styles

Author: fulleni

android/app/src/main/res/drawable-hdpi/splash.png

@@ -3,4 +3,7 @@
     <item>
         <bitmap android:gravity="fill" android:src="@drawable/background"/>
     </item>
+    <item>
+        <bitmap android:gravity="center" android:src="@drawable/splash"/>
+    </item>
 </layer-list>

android/app/src/main/res/drawable-mdpi/splash.png

@@ -3,4 +3,7 @@
     <item>
         <bitmap android:gravity="fill" android:src="@drawable/background"/>
     </item>
+    <item>
+        <bitmap android:gravity="center" android:src="@drawable/splash"/>
+    </item>
 </layer-list>

android/app/src/main/res/drawable-night-hdpi/splash.png

@@ -3,4 +3,7 @@
     <item>
         <bitmap android:gravity="fill" android:src="@drawable/background"/>
     </item>
+    <item>
+        <bitmap android:gravity="center" android:src="@drawable/splash"/>
+    </item>
 </layer-list>