feat(settings): choose between Save As and saving into a folder

Downloading a selection of twenty files meant twenty Save As dialogs,
which is the wrong answer for the feature that had just been built.
Settings → Downloads now offers saving automatically, and that is the
default; asking every time stays available for people who want it.

The correction worth recording: a web page cannot be given a filesystem
path and cannot read one either. There is no ~/Downloads to configure and
nothing to type, on any operating system — which is also why none of this
will need changing on Windows. What a browser grants is a handle to a
folder the user picked in a dialog, so that is what the setting keeps:
picked once, stored in IndexedDB, re-confirmed once a session because the
grant comes back as a claim rather than a permission. Where no folder has
been granted, and in Firefox and Safari where none can be, files go to
the browser's own download folder — which on most machines is the folder
that was meant all along.

Automatic saving has one risk a dialog does not: it can silently replace
a file. It does not — a taken name gets a suffix before the extension,
`clip (2).mp4`, so a download folder does not fill up with files the
system no longer recognises. That, and the default, are what
test_downloads.py pins.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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