feat(rules): every Maildir folder in the Folder dropdown

The dropdown was built from config, which names one subtree per account
and nothing below it, so it offered five entries and no way to say
Drafts or Sent. A rule wants to target those as often as a whole
account.

NotmuchWorker gains requestFolders/foldersReady, walking the tree from
notmuch_database_get_path() and listing every directory holding cur/.
It belongs there because the database root is notmuch's database.path
and the worker owns the only handle that can answer for it; putting the
root in config would be the second source of truth the design refuses.

From the disk rather than from the index: a folder mbsync created and
nothing has landed in yet is still a folder a rule may target, and a
list derived from indexed message paths would not offer it.

The two tests build their own fixture rather than extending the shared
one, which needs a nested folder and would otherwise move seven count
assertions in unrelated tests. Mutation-checked: flattening the walk to
non-recursive fails the listing test.

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

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