feat(placeholder): count sent mail and drafts on the blank pane

Item 67. The pane counted unread, flagged and inbox from three fixed
tag: queries. Sent and drafts cannot join that list as tags: tag:draft
counts 0 against a real database and no draft-ish tag exists in it at
all, so a tag-based line would be a permanent zero that reads as working
code. Both are composed from each account's folder keys instead, the
same way the Sent view already composes its query.

The drafts key was parsed and documented as unused in v1. Composing
drafts is still v2; counting them is not, so Account::draftsQuery() and
Config::allDraftsQuery() now mirror the sent pair. The shared body moved
into folderQuery() and joinAccountQueries(), so the load-bearing quoting
(a provider nests both folders under a bracketed parent, and [ and ] are
Xapian syntax) and the bare-"or" guard exist once rather than once per
folder type.

The fixed array is gone rather than extended. It held queries and labels
in two lists indexed in parallel, which is a hazard that grows with the
list: an entry inserted in one and not the other prints a real number
against the wrong name and looks entirely plausible. placeholderLines()
carries each query beside the callable that labels it, so the two cannot
drift, and the count reply stays paired by position as the worker
requires.

A line is omitted when no account configures that folder rather than
shown as 0, following item 63: a missing folder is a real configuration,
and "0 sent" claims the user has sent nothing.

Measured against the real config: 4 sent terms over 601 threads, 5
drafts terms over 3, the extra drafts term coming from the one account
that configures drafts and no sent, which is what proves the two are
collected independently.

Four tests here and four in test_config, mutation-checked at three
points: dropping the drafts line, an off-by-one in the label pairing,
and removing the -1 guard for an uncountable query. Each mutation fails
a test.

1 files changed, 6 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 22a6159..9d872ec 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,12 @@ point at which they are stable.
 - Sent mail is shown as a flat list rather than as threads, and the cards name
   the **recipients** instead of the sender, which is you on every row.
   Selecting one opens what you sent, not the conversation your message started.
+- The blank message pane counts **sent mail and drafts** beside unread, flagged
+  and inbox. Both are composed from each account's `sent` and `drafts` folder
+  rather than from a tag, so they follow the same per-account configuration the
+  Sent view uses. A line is absent entirely when no account configures that
+  folder, rather than reading 0. The `drafts` key was already accepted and
+  documented as unused; it now has an effect.
 - `[general] date_format`, an optional pattern for the date on a thread card.
   Absent or empty keeps the system locale's short format, which is unchanged
   and remains the default. A pattern containing no date or time field is