feat(desktop): customize macOS DMG install window (#13563)

* feat(desktop): add Retina DMG background tooling

* feat(desktop): customize the macOS DMG layout

* ci(desktop): validate DMG background assets

* fix(desktop): adjust DMG Applications icon position

* ci(desktop): drop redundant DMG artwork validation from publish workflow

Tauri's beforeBuildCommand already runs dmg:background (with its own
validation) at the start of the build/sign/notarize step, and the
release/beta config overlays do not override the build section, so this
step duplicated work the publish job performs anyway. PR-time coverage
lives in desktop-test.yml.

Co-authored-by: Saoud Rizwan <saoudrizwan@users.noreply.github.com>

---------

Co-authored-by: Saoud Rizwan <7799382+saoudrizwan@users.noreply.github.com>
Co-authored-by: Saoud Rizwan <saoudrizwan@users.noreply.github.com>

.github/workflows/desktop-test.yml

@@ -0,0 +1,50 @@
+name: desktop-test
+
+on:
+  push:
+    branches:
+      - main
+      - desktop-experimental
+    paths:
+      - "apps/examples/desktop-app/package.json"
+      - "apps/examples/desktop-app/scripts/dmg-background.ts"
+      - "apps/examples/desktop-app/scripts/dmg-background.test.ts"
+      - "apps/examples/desktop-app/src-tauri/dmg/background.png"
+      - "apps/examples/desktop-app/src-tauri/dmg/background@2x.png"
+      - ".github/workflows/desktop-test.yml"
+  pull_request:
+    branches:
+      - main
+      - desktop-experimental
+    paths:
+      - "apps/examples/desktop-app/package.json"
+      - "apps/examples/desktop-app/scripts/dmg-background.ts"
+      - "apps/examples/desktop-app/scripts/dmg-background.test.ts"
+      - "apps/examples/desktop-app/src-tauri/dmg/background.png"
+      - "apps/examples/desktop-app/src-tauri/dmg/background@2x.png"
+      - ".github/workflows/desktop-test.yml"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  dmg-background:
+    name: Test DMG background tooling
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: apps/examples/desktop-app
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Setup Bun
+        uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: "1.3.13"
+
+      # The suite only uses Bun/Node built-ins and committed artwork, so it does
+      # not need a workspace dependency install or macOS runner.
+      - name: Test DMG background tooling
+        run: bun run test:dmg-background