Replaces the default Flutter logo with the project's rainbow-rings icon
(icon.svg) across all platforms:
- Android: regenerate all mipmap ic_launcher.png sizes (mdpi–xxxhdpi)
from icon.svg using Inkscape
- Linux: add sharedinbox.png (512×512), install it next to the binary
via CMakeLists.txt, and set it as the GTK window icon at runtime
- Adds icon.png (1024×1024 source raster) committed for reference
- Adds flutter_launcher_icons dev dep + flutter_icons config so icons
can be regenerated via `task generate-icons` in future
- Adds librsvg to flake.nix so rsvg-convert is available in nix develop
Closes#451
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
