Fix NSIS installer build: absolute paths via defines, space-free installed exe, BMP ICO

- makensis resolves relative -DICON/-DARTIFACT against the script directory,
  so CI now passes $PWD-based absolute paths (reproduced locally in the same
  ubuntu:24.04 container; installer builds, 7z shows step-line.exe +
  uninstall.exe)
- Install as step-line.exe (no spaces: NSIS /oname splits on whitespace)
- OutFile overridable via -DOUT
- tools/generate_textures.py icon.ico now produced via ImageMagick with
  classic BMP entries (NSIS-compatible); PIL PNG-compressed ICOs rejected by
  NSIS

tools/generate_textures.py

@@ -306,12 +306,30 @@ def app_icon():
     icon("icon.png", badge, size=1024)
     icon("icon.ico", badge, size=256)
 
-    # ICO for Windows. Sizes stay at 16/32/48 so every entry uses classic BMP
-    # (PNG-compressed entries, used by PIL at >=256px, break some NSIS builds).
-    src = Image.open(os.path.join(ASSETS, "icon.png")).convert("RGBA")
+    # ICO for Windows. Must be BMP-entry based: PIL's ICO serializer emits
+    # PNG-compressed entries which NSIS' icon loader rejects. ImageMagick
+    # writes classic BMP entries; fall back to PIL only if ImageMagick is gone
+    # (flag it so a human notices).
+    import subprocess
+    import sys
     ico_path = os.path.join(ASSETS, "icon.ico")
-    src.save(ico_path, format="ICO", sizes=[(48, 48), (32, 32), (16, 16)])
-    print("wrote assets/textures/icon.ico (BMP 16/32/48)")
+    src_png = os.path.join(ASSETS, "icon.png")
+    ran = False
+    for cmd in (
+        ["magick", src_png, "-background", "none", "-define", "icon:auto-resize=48,32,16", ico_path],
+        ["convert", src_png, "-background", "none", "-define", "icon:auto-resize=48,32,16", ico_path],
+    ):
+        try:
+            subprocess.run(cmd, check=True, capture_output=True)
+            ran = True
+            break
+        except (FileNotFoundError, subprocess.CalledProcessError):
+            continue
+    if ran:
+        print("wrote assets/textures/icon.ico (BMP 16/32/48 via ImageMagick)")
+    else:
+        print("WARNING: ImageMagick not found; writing PIL ICO (may be PNG-compressed)")
+        Image.open(src_png).convert("RGBA").save(ico_path, format="ICO", sizes=[(48, 48), (32, 32), (16, 16)])
 
 
 def cards():