Use a transparent Dock icon

[Dezoff-max]

What changed

• regenerated every AppIcon size from the existing transparent high-resolution broom asset
• removed the baked rounded gray background from the packaged icon
• set BrandIcon as the runtime NSApplication icon so macOS does not add its fallback Dock plate

Why

The menu bar used a transparent image, while the Dock icon came from a background-backed AppIcon and macOS continued normalizing the transparent ICNS onto a gray plate.

Validation

• all seven AppIcon sizes have the expected dimensions and alpha channel
• the compiled AppIcon.icns contains the transparent broom
• ./script/build_and_run.sh --verify passed after clearing transient build metadata
• 15 CleanMacCore tests passed
• the running Dock icon was visually verified without a background