3e2da2bdf8
Closes #451 ## What changed Replaces the default Flutter blue logo with the project's rainbow-rings `icon.svg` on all supported platforms. **Android** — all five mipmap densities regenerated (`mdpi` 48px through `xxxhdpi` 192px). **Linux** — `linux/sharedinbox.png` (512×512) added, installed next to the binary via `CMakeLists.txt`, and set as the GTK window icon via `gtk_window_set_icon_from_file` in `my_application.cc`. **Tooling** — `icon.png` (1024×1024 source raster) committed; `flutter_launcher_icons` added as dev dep with a `flutter_icons` config block; `task generate-icons` added to `Taskfile.yml` for future regeneration; `librsvg` added to `flake.nix` so `rsvg-convert` is available inside `nix develop`. ## How verified Icons were generated with Inkscape from `icon.svg` and visually confirmed (rainbow-rings design appears correctly at all sizes). The `playstore/icon.png` was already correct and unchanged. Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de> Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/459