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:

flake.nix

@@ -100,6 +100,7 @@
             ]))  # used by stalwart-dev/start and deploy_playstore.py
             fgj      # Codeberg/Forgejo CLI (like gh for GitHub)
             skopeo   # inspect OCI image manifests without pulling layers (used by check-ci-images)
+            librsvg  # rsvg-convert — SVG→PNG for generate-icons task
           ]);
 
           shellHook = ''

linux/CMakeLists.txt

@@ -102,3 +102,7 @@ if(NOT CMAKE_BUILD_TYPE MATCHES "Debug")
   install(FILES "${AOT_LIBRARY}" DESTINATION "${INSTALL_BUNDLE_LIB_DIR}"
     COMPONENT Runtime)
 endif()
+
+install(FILES "${CMAKE_CURRENT_SOURCE_DIR}/sharedinbox.png"
+  DESTINATION "${CMAKE_INSTALL_PREFIX}"
+  COMPONENT Runtime)

linux/my_application.cc

@@ -31,6 +31,8 @@ static void my_application_activate(GApplication* application) {
 
   fl_register_plugins(FL_PLUGIN_REGISTRY(view));
 
+  gtk_window_set_icon_from_file(window, "sharedinbox.png", nullptr);
+
   // Show AFTER adding FlView so GTK's first layout pass allocates the full
   // window content area (1280×800) to FlView, not the default 1×1.
   gtk_widget_show_all(GTK_WIDGET(window));

pubspec.yaml

@@ -81,6 +81,15 @@ dev_dependencies:
   sqlite3: ^3.1.5  # used directly in test/unit/db_test_helper.dart; 3.x required for Database.close()
   url_launcher_platform_interface: ^2.3.2
   plugin_platform_interface: ^2.1.8
+  flutter_launcher_icons: ^0.13.1
+
+flutter_icons:
+  android: "ic_launcher"
+  ios: false
+  image_path: "icon.png"
+  linux:
+    generate: true
+    image_path: "icon.png"
 
 flutter:
   uses-material-design: true