Support Dropbox App folder access instead of Full Dropbox access

Currently the Dropbox adapter requests full-Dropbox-access scopes (`files.content.write`, `files.content.read`, `files.metadata.write`, `account_info.read`), and `bookmark_file` writes relative to the Dropbox root rather than an `Apps/floccus/` app folder. In practice this means Floccus's Dropbox app can read/write anywhere in the user's Dropbox, not just its own sync file.

This was raised as a follow-up in #2291 by @tan-x-y after the scope-error fix there:
> One additional thought that might still be worthwhile is limiting the Dropbox permission scope to App folder instead of full access. Using App folder would simplify maintenance and keep all saved XBEL files neatly contained under Apps (e.g., in a floccus subfolder), similar to how mainstream userscript managers handle their Dropbox backups. This would also reduce clutter for users and lower the risk associated with granting full Dropbox access.

Requesting this as its own item since #2291 is closed under an unrelated title. Would it be feasible to register (or offer as an alternate app ID) a Dropbox app with "App folder" access type, confined to `Apps/floccus/`, for users who don't want to grant full-Dropbox scope?

Start by locating the Dropbox adapter and the bookmark_file implementation, then inspect how the current full-Dropbox scopes and app ID are configured. Determine whether an App-folder Dropbox app or alternate app ID can be supported without breaking existing users. Done means saved XBEL files use the confined Apps/floccus/ location and the requested permissions no longer grant unnecessary full-Dropbox access.

Written by the indexing model from the issue text.