feat(account): add support for PNG avatars

[McBaumwolle]

Description
This PR updates the account avatar logic to support both .jpg and .png file extensions. Previously, all avatars were forced to use the .jpg extension, which caused issues when someone uploaded PNGs, especially those with transparency (I tried some for my main email and did not understand why it failed at first, or rather did not show the images in the picker).

Changes:
Dynamic extension: LocalAvatarImageDataSource now detects if the source image is a PNG and saves it with the correct .png extension.

• Smart cleanup: the delete() function now checks for and removes both .jpg and .png files for the given account. This ensures that, when a user switches from a JPEG avatar to a PNG avatar (or vice versa), the old file is deleted and does not remain orphaned.
• Tests: updated LocalAvatarImageDataSourceTest to verify PNG support and ensure the clean-up logic works for multiple file types.

Fixes:
#10501

[wmontwe]

Thank your for adding support for PNG avatars!

You also need to patch the UpdateAvatarImage use case as it checks the mime type and rejects non JPEG files. Additionally I would change the imagePicker to use OpenDocument to allow you to define a list of allowed file types.

[McBaumwolle]

Thanks! Maybe I completely forgot about that. It should work now with ActivityResultContracts.OpenDocument() - however, as the name suggests it now opens the document picker and not the "fancy" new image picker. In my opinion it makes no difference tho. I tested it on my Pixel again and it works as expected, and rejects (or simply filters out) anything that is not .png or .jpg. I also adjusted the test (LocalAvatarImageDataSourceTest.kt) for these.