Add placeholder app icon and wire it into the Windows build

Generate build/icon.svg — a white download glyph on the teal brand square,
mirroring the in-app brand mark (Onboarding.tsx) — and rasterise it to a
multi-size build/icon.ico (256/128/64/48/32/16) via ImageMagick. Uncomment
win.icon in electron-builder.yml so installers stop falling back to the default
Electron icon. Marks CODE-AUDIT.md M3 placeholder-done; a designed asset is
still wanted before v1.0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

electron-builder.yml

@@ -42,10 +42,11 @@ win:
     - portable
   # Never request admin elevation.
   requestedExecutionLevel: asInvoker
-  # TODO(release, audit M3): drop a 256×256 (multi-size) build/icon.ico in place
-  # and uncomment the next line before cutting v1.0. Until then electron-builder
-  # falls back to its default Electron icon, which is fine for dev/test builds.
-  # icon: build/icon.ico
+  # Placeholder icon (audit M3): white download glyph on the teal brand square,
+  # mirroring the in-app brand mark. Generated from build/icon.svg as a multi-size
+  # .ico (256/128/64/48/32/16). Replace with a designed asset before v1.0 — re-run
+  #   magick -background none build/icon.svg -define icon:auto-resize=256,128,64,48,32,16 build/icon.ico
+  icon: build/icon.ico
 
 portable:
   artifactName: ${productName}-${version}-portable.${ext}