fix(companion): build native 64-bit NSIS installer

The NSIS installer was a 32-bit PE, causing $PROGRAMFILES to resolve
to "Program Files (x86)" on 64-bit Windows. Adding "Target amd64-unicode"
produces a native 64-bit installer where $PROGRAMFILES correctly points
to "Program Files".

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: raphaelcoeffic

companion/targets/windows/companion.nsi.in

@@ -18,6 +18,9 @@
   Name "@PROJECT_NAME@ Companion @VERSION_FAMILY@"
   OutFile "companion-windows-@VERSION@.exe"
 
+  ;Build a native 64-bit installer
+  Target amd64-unicode
+
   ;Default installation folder
   InstallDir "$PROGRAMFILES\@PROJECT_NAME@\Companion @VERSION_FAMILY@"