Replace app icon with Flutter icon for Design Toolbox

[Tayebsed93]

Context :

The current version of the Design Toolbox app uses the default native icon.
To improve visual identification and better distinguish between the native and Flutter versions of the app, we should replace the current icon with a Flutter-branded one.

• Replace the existing native icon in the project
• Test the new icon on multiple platforms (iOS/Android if applicable)

[Tayebsed93]

📣 New Firebase App Distribution ALPHA upload available 🚀 (Android)

Please, get the new build from App Tester app, or contact the project maintainers (@Tayebsed93) to get access.

Display name: DesignToolbox (alpha) (300)
Version: 0.5.0
Build type: ALPHA
Build details (GitHub): 268
cc @B3nz01d (product owner)
cc @Tayebsed93 @AhmedAmineZr @nouha06 (dev team)
cc @jerome-regnier (design team)

[Tayebsed93]

📣 New TestFlight ALPHA upload available 🚀 (iOS)

Please, get the new build from TestFlight app, or contact the project maintainers (@Tayebsed93) to get access.

Display name: Design Toolbox Flutter (300)
Version: 0.5.0
Build type: ALPHA
Build details (GitHub): 268
cc @B3nz01d (product owner)
cc @Tayebsed93 @AhmedAmineZr @nouha06 (dev team)
cc @jerome-regnier (design team)

[Tayebsed93]

📣 New Firebase App Distribution BETA upload available 🚀 (Android)

Please, get the new build from App Tester app, or contact the project maintainers (@Tayebsed93) to get access.

Display name: DesignToolbox (beta) (301)
Version: 0.5.0
Build type: BETA
Build details (GitHub): 268
cc @B3nz01d (product owner)
cc @Tayebsed93 @nouha06 @AhmedAmineZr (dev team)
cc @jerome-regnier (design team)

[Tayebsed93]

📣 New TestFlight BETA upload available 🚀 (iOS)

Please, get the new build from TestFlight app, or contact the project maintainers (@Tayebsed93) to get access.

Display name: Design Toolbox Flutter (beta) (301)
Version: 0.5.0
Build type: BETA
Build details (GitHub): 268
cc @B3nz01d (product owner)
cc @Tayebsed93 @AhmedAmineZr @nouha06 (dev team)
cc @jerome-regnier (design team)