clien: pdf-gen: load logo externally; add prepare step * Change the way how the logo to be placed in the pdf is loaded: It can be configured in the `.env.local´ file, otherwise a placeholder will be loaded from the static assets. The placeholder has not been placed in `assets` because it won't be needed if a real logo has been specified and thus should not compiled in by default. * Add a mechanism to do asynchronous preparations before rendering. This is needed for the completeion of the Image() loading, but it also shows how asynchronous preparational step can be added in principle. * Disable the start button when the current rendering is in progress.