feat: add Nova application icon

[bigduu]

Summary

• add the approved 1024×1024 Nova “Star-Core Pulse” artwork as the canonical app icon source
• generate a standard ten-representation macOS iconset during app packaging and embed Nova.icns before code signing
• declare branded Chrome extension and toolbar icons at 16/24/32/48/128px
• replace the temporary letter mark in the Chrome popup with the Nova icon
• validate PNG format, dimensions, RGBA channels, iconset mapping, plist wiring, bundle signing order, and archive contents

Verification

• npm --prefix chrome/extension run check
• Chrome extension tests: 122/122
• scripts/test-release-workflow.sh
• bash -n packaging/macos/package-development-app.sh scripts/test-release-workflow.sh
• git diff --check
• exact remote tree SHA matches the locally reviewed tree

The hermetic Linux release test mocks sips and iconutil; the real macOS release job remains fail-closed and performs the actual icon conversion before signing the app bundle.