ci: pre-commit branding BMPs, remove broken ImageMagick CI steps

Chocolatey ImageMagick on Windows runners lacks PNG codec delegates,
causing prepare-branding.ps1 to fail with:
  magick.exe: no decode delegate for this image format

Fix: generate header.bmp (150x57) and welcome.bmp (164x314) on macOS
using Homebrew ImageMagick from img/linux-collection.png, commit them
to the repo, and remove the Install ImageMagick and Generate installer
branding assets CI steps entirely. BMPs will be regenerated locally
when the source image changes.

Author: eoinjordan

.github/workflows/build-windows-installer.yml

@@ -55,14 +55,6 @@ jobs:
         shell: pwsh
         run: choco install nsis --no-progress -y
 
-      - name: Install ImageMagick
-        shell: pwsh
-        run: choco install imagemagick --no-progress -y
-
-      - name: Generate installer branding assets
-        shell: pwsh
-        run: windows-installer/prepare-branding.ps1
-
       - name: Build NSIS installer
         shell: pwsh
         run: |

windows-installer/.gitignore

@@ -1,3 +1,4 @@
 staging/
 output/
-branding/
+# Only ignore generated sub-folder; header.bmp / welcome.bmp are pre-built and committed.
+branding/generated/