feat(android): add app logo as launcher icon

Generate mipmap ic_launcher.png at all standard densities (mdpi→xxxhdpi)
from logo.png, cropped to bounding box and scaled to 88% of the tile on
a white background (legacy fallback for API < 26).

For API 26+ provide an adaptive icon (ic_launcher.xml / ic_launcher_round.xml)
with white background and a foreground PNG where the logo occupies 56% of
the 108dp canvas — within the safe zone, fully visible on every launcher
shape without clipping.

src/App.Android/MainApplication.cs

@@ -10,7 +10,8 @@ namespace PalladiumWallet.Mobile;
 // In Avalonia 12 l'AppBuilder Android si configura nella sottoclasse Application
 // (AvaloniaAndroidApplication<TApp>), non più nell'Activity. allowBackup=false:
 // il file wallet cifrato/seed non deve finire nei backup cloud automatici.
-[Application(Label = "Palladium Wallet", AllowBackup = false)]
+[Application(Label = "Palladium Wallet", AllowBackup = false,
+             Icon = "@mipmap/ic_launcher", RoundIcon = "@mipmap/ic_launcher_round")]
 public class MainApplication : AvaloniaAndroidApplication<global::PalladiumWallet.App.App>
 {
     public MainApplication(IntPtr javaReference, JniHandleOwnership transfer)

src/App.Android/Resources/mipmap-anydpi-v26/ic_launcher.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
+    <background android:drawable="@color/ic_launcher_background"/>
+    <foreground android:drawable="@drawable/ic_launcher_foreground"/>
+</adaptive-icon>

src/App.Android/Resources/mipmap-anydpi-v26/ic_launcher_round.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
+    <background android:drawable="@color/ic_launcher_background"/>
+    <foreground android:drawable="@drawable/ic_launcher_foreground"/>
+</adaptive-icon>

src/App.Android/Resources/values/ic_launcher_background.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <color name="ic_launcher_background">#FFFFFF</color>
+</resources>