Fix "Unknown image format" error when showing or updating profiles

wx only decodes the image formats its own handlers support. Mastodon
instances commonly serve avatars and headers as WebP, so wx.Image failed
and raised a modal "Unknown image format" dialog, which blocked the user
from updating their profile or from reading someone's profile.

Images are now decoded with Pillow and their raw pixel data handed over
to wx, through a new mysc.image_utils module shared by the Mastodon and
Bluesky profile dialogs. Undecodable or unreachable images log the
problem and leave an empty placeholder instead of raising.

Also accept WebP and JPEG files in the header/avatar file pickers, warn
the user with a common message dialog when the picked file cannot be
read, and fix the parentless wx.StaticBitmap placeholders that were
created when a profile image could not be downloaded.

doc/changelog.md

@@ -53,6 +53,7 @@ This version introduces comprehensive support for the AT Protocol (ATProto), ena
 * Mastodon:
     * Added support for sending quoted posts! You can now quote other users' posts from the context menu or the new Boost dialog. ([#860](https://github.com/mcv-software/twblue/issues/860))
     * Fixed an issue where HTML entities were not decoded when editing a post. ([#893](https://github.com/mcv-software/twblue/issues/893))
+    * Fixed an error that displayed an "Unknown image format" dialog when viewing someone's profile or updating your own, if the avatar or header was served in a format TWBlue could not read, such as WebP. ([#977](https://github.com/mcv-software/twblue/issues/977))
 
 ## Changes in version 2026.01.13