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>

build/icon.svg

@@ -0,0 +1,12 @@
+<svg width="256" height="256" viewBox="0 0 256 256" xmlns="http://www.w3.org/2000/svg">
+  <!-- Placeholder app icon (audit M3). Mirrors the in-app brand mark in
+       Onboarding.tsx: a white download arrow on a rounded square in the default
+       "teal" accent (theme.ts teal[80] = colorBrandBackground in light mode).
+       Replace with a designed asset before v1.0; regenerate icon.ico from this. -->
+  <rect x="0" y="0" width="256" height="256" rx="52" ry="52" fill="#148185"/>
+  <g fill="#ffffff">
+    <rect x="112" y="46" width="32" height="88" rx="12" ry="12"/>
+    <polygon points="82,114 174,114 128,176"/>
+    <rect x="66" y="190" width="124" height="20" rx="10" ry="10"/>
+  </g>
+</svg>