[Feature Request] Persist the "Use formatted filename" choice for downloads

Description Current state Since #12095, the document detail view has a "Use formatted filename" option in the download dropdown. Thanks for that — it solves the naming itself nicely. However, the choice is not remembered. It resets after every download, and even when navigating to another document and back within the same session. For anyone who regularly sends documents out, this means ticking the box every single time — and re-downloading whenever it was forgotten. Feature request Allow the choice to persist instead of resetting. Any of these would solve it, roughly in order of preference: 1. Persist the checkbox state per user in the UI settings, like other frontend preferences. 2. A user-level setting (Settings → General), e.g. "Use formatted filename by default when downloading". 3. An instance-level variable, e.g. PAPERLESS_DOWNLOAD_USE_FORMATTED_FILENAME, defaulting to false so nothing changes for existing users. Option 1 alone would alr…