feat(settings): support avatar upload and robust avatar deletion

[tyreseluo]

Summary

• Add avatar upload support in Account Settings on desktop platforms.
• Introduce MatrixRequest::UploadAvatar flow to upload selected PNG/JPEG files.
• Add compatibility fallback for avatar deletion when homeserver returns M_UNRECOGNIZED on delete-avatar endpoint.
• Show clear "not yet supported" notifications for avatar upload/delete on non-desktop platforms.

Changes

• Update settings UI logic to:
• Open a file picker for avatar upload on desktop.
• Trigger upload spinner/action state while upload is in progress.
• Show platform-specific warning on mobile for avatar actions.
• Update sliding sync worker to:
• Handle UploadAvatar by reading local file, validating PNG/JPEG, and calling account avatar upload.
• Retry avatar deletion via fallback request when standard deletion path fails with M_UNRECOGNIZED.
• Add required dependencies:
• mime for content type handling.
• rfd (desktop-only target dependency) for file picker support.

Validation

• cargo check passes.
• Manual behavior covered:
• Avatar can be uploaded from settings on desktop.
• Avatar deletion gracefully handles servers that reject the newer deletion endpoint.
• Mobile avatar actions show explicit not-supported notifications.