Add Hash Wallet app icon master + enable flutter_launcher_icons

- New assets/images/app_logo.png — 1024x1024 master with white # on
  green (#1a5c38) background. Source SVG checked in alongside as
  hash_wallet_app_icon_source.svg.
- Enable flutter_launcher_icons dependency (was commented out) and rename
  the legacy 'flutter_icons' config key to the modern 'flutter_launcher_icons'
  key. Added linux + windows generation; remove_alpha_ios per Apple
  requirement that App Store icons not have alpha.

After pub get, run 'flutter pub run flutter_launcher_icons' to populate
the per-platform icon variants (iOS appiconset, Android mipmaps, macOS
appiconset, etc.). Splash / launch images are still Cake's defaults; that's
a follow-up.

assets/images/hash_wallet_app_icon_source.svg

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1024 1024" width="1024" height="1024">
+  <title>Hash Wallet app icon</title>
+  <!-- Solid green background. Color matches the brand accent in
+       hash_wallet_logos.svg (#1a5c38). iOS/Android round the corners
+       automatically; we ship a square fill. -->
+  <rect width="1024" height="1024" fill="#1a5c38"/>
+  <!-- # mark, white, centered. ui-monospace falls back to the platform's
+       default monospace; the rasterizer bakes this into a PNG so font
+       availability on user devices doesn't matter. -->
+  <text x="512" y="512"
+        text-anchor="middle"
+        dominant-baseline="central"
+        font-family="ui-monospace, 'JetBrains Mono', 'Courier New', monospace"
+        font-size="720"
+        font-weight="700"
+        fill="#ffffff">#</text>
+</svg>

pubspec_base.yaml

@@ -176,8 +176,7 @@ dev_dependencies:
   mobx_codegen: ^2.1.1
   build_resolvers: ^2.4.4
   hive_generator: ^2.0.1
-  # flutter_launcher_icons: ^0.11.0
-  # check flutter_launcher_icons for usage
+  flutter_launcher_icons: ^0.13.1
   pedantic: ^1.8.0
   # replace https://github.com/dart-lang/lints#migrating-from-packagepedantic
   translator:
@@ -228,13 +227,21 @@ dependency_overrides:
   flutter_rust_bridge: 2.11.1
   flutter_secure_storage_windows: 3.1.2
 
-flutter_icons:
+flutter_launcher_icons:
   image_path: "assets/images/app_logo.png"
   android: true
   ios: true
+  remove_alpha_ios: true
   macos:
     generate: true
     image_path: "assets/images/app_logo.png"
+  linux:
+    generate: true
+    image_path: "assets/images/app_logo.png"
+  windows:
+    generate: true
+    image_path: "assets/images/app_logo.png"
+    icon_size: 256
 
 flutter:
   uses-material-design: true