feat: use icon.svg as app icon for Android and Linux (#459)

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

Taskfile.yml

@@ -58,6 +58,14 @@ tasks:
     cmds:
       - echo "Setup complete."
 
+  generate-icons:
+    desc: Rasterise icon.svg → icon.png and regenerate all platform launcher icons
+    deps: [_pub-get]
+    cmds:
+      - rsvg-convert -w 1024 -h 1024 icon.svg -o icon.png
+      - rsvg-convert -w 512  -h 512  icon.svg -o playstore/icon.png
+      - fvm flutter pub run flutter_launcher_icons
+
   generate-changelog:
     desc: Generate assets/changelog.txt from git history
     cmds: