feat(ui): fill the blank message pane with a branded placeholder

An empty right pane said nothing, and multi-select made it a routine
sight. It now carries the wordmark, thread counts that run their query
when clicked, and a sync line that appears only when something needs
attention.

Rendered into the existing web view as a third document shape, so there
is one document path and one set of security rules. The brand palette is
a deliberate exception to deriving colours from the desktop theme, since
a logo is brand rather than chrome; the theme still picks which of the
two sets is used.

Counts refresh when the pane is about to show rather than in the
background: one goes stale the moment a tag is edited, and refreshing one
nobody is looking at is work for nothing. A generation counter discards a
superseded reply, and a late answer cannot repaint over an opened thread.

The helper lines are real links because JavaScript is off in this
profile. The handler is gated on the placeholder actually being
displayed, so the same URL inside a message body is dropped: a stranger's
mail must not drive the thread list, even to run a harmless query.

Three defects found while building, all silent:

- Every CSS percentage was invalid. QString::arg does not collapse "%%"
  into "%", so the document carried "50%%" and the browser dropped each
  declaration holding one, disabling the mask, the glow and both radial
  gradients while still rendering something plausible. Substitution is by
  named token now, which cannot collide with a percent sign.
- A geometry probe endorsed the layout while that was live, because it
  measured only properties without percentages.
- The font test passed against a build with one face missing, since the
  other satisfied both of its checks on its own.

The mockup's light values needed correcting against a real pane: the grid
vanished at a 2% luminance step on white, and the glow subtracts light
there rather than adding it, washing the pane. Strength only, not hue.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/notmuchworker.h b/src/notmuchworker.h
index 96e75f8..05c1d3a 100644
--- a/src/notmuchworker.h
+++ b/src/notmuchworker.h
@@ -74,6 +74,15 @@ public slots:
     /// after a sync, and after a tag mutation introduces an unknown tag.
     void requestAllTags(quint64 generation);
 
+    /// Thread counts for the placeholder pane's helper lines, one per query,
+    /// answered in the order asked. Counts rather than results: the pane says
+    /// how much there is, and clicking a line runs the query properly.
+    ///
+    /// Requested when the pane is about to go blank rather than kept fresh in
+    /// the background. A count goes stale the moment a tag is edited, and
+    /// refreshing one nobody is looking at is work for nothing.
+    void requestCounts(const QStringList &queries, quint64 generation);
+
 signals:
     void threadsReady(const QVector<ThreadSummary> &threads, quint64 generation);
     void queryFinished(int totalThreads, quint64 generation);
@@ -81,6 +90,11 @@ signals:
     void tagsApplied(const TagChange &change);
     void allTagsReady(const QStringList &tags, quint64 generation);
 
+    /// One entry per requested query, in the order they were asked for. A query
+    /// notmuch rejects yields -1 rather than dropping the entry, so the
+    /// positional correspondence the caller relies on always holds.
+    void countsReady(const QVector<int> &counts, quint64 generation);
+
     void errorOccurred(const QString &message);
 
 private: