fix: improve banners

[connyduck]

Problems with the existing banners:

• The icons in the background are not tiled correctly (there is more space on the right side of the banner than on the left side)
• The transparent banner has white transparency artifacts (it probably was created by converting white to transparency) that make it look weird when rendered over a dark background, e.g. when Fluffychat is in dark mode.

I corrected the background tiles and changed the transparent banner to make sure it looks good no matter which background it is displayed over.

Otherwise I tried to stay as close to the existing banners as possible.

The new banner over white background:

The new banner over dark background:

The banner inside the Android app:

• Code formatting and import sorting has been done with dart format lib/ test/ and dart run import_sorter:main --no-comments
• The commit message uses the format of Conventional Commits
• The commit message describes what has been changed, why it has been changed and how it has been changed
• Every new feature or change of the design/GUI is linked to an approved design proposal in an issue
• Every new feature in the app or the build system has a strategy how this will be tested and maintained from now on for every release, e.g. a volunteer who takes over maintainership

Pull Request has been tested on:

• Android
• iOS
• Browser (Chromium based)
• Browser (Firefox based)
• Browser (WebKit based)
• Desktop Linux
• Desktop Windows
• Desktop macOS