feat(ui): make the toolbar icon size configurable

Follow-up to item 56. With the toolbar now following the desktop's
button style, an "icon only" desktop makes the icon the whole control,
and this style reports PM_ToolBarIconSize as 16px, which is a small
target for a button with no text beside it.

A [general] toolbar_icon_size key, 16 to 64, defaulting to 24 rather
than to the style's own metric. Setting it to 16 restores the theme's
value.

Clamped and reported, unlike message_zoom, which documents a 0.5 to 3.0
range in the README and enforces none of it. Both ends here break the
UI that would be used to fix them: too small is an invisible icon, too
large is a toolbar taller than the window.

The unenforced message_zoom range is recorded as item 58 rather than
fixed here, since it is a separate defect that predates this change.

Also documents in the README that saved-query button labels are the key
names from the user's own [queries] section, which is why the "Flagged"
button still read that way after the action was renamed: it is a user's
query name, not a string this code owns. The sample config now shows
`Important = tag:flagged` to teach the wording the UI uses.

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

3 files changed, 59 insertions, 0 deletions

diff --git a/src/config.cpp b/src/config.cpp
index 12c5632..b54d5c6 100644
--- a/src/config.cpp
+++ b/src/config.cpp
@@ -24,6 +24,16 @@
 #include <QSettings>
 #include <QStandardPaths>
 
+namespace {
+
+/// Bounds for [general] toolbar_icon_size. 16 is the smallest size the icon
+/// themes actually ship art for, and is what this desktop's style reports;
+/// above 64 the toolbar is taller than the thread rows it sits over.
+constexpr int kMinToolbarIconSize = 16;
+constexpr int kMaxToolbarIconSize = 64;
+
+} // namespace
+
 QString Account::scopedQuery(const QString &query) const
 {
     const QString prefix = QStringLiteral("path:\"%1/**\"").arg(maildir);
@@ -93,6 +103,34 @@ void Config::load(const QString &path)
     m_completionOnFocus =
         settings.value(QStringLiteral("completion_on_focus"), false).toBool();
 
+    // Clamped, unlike message_zoom above, which documents a 0.5 to 3.0 range in
+    // the README and enforces none of it. Both ends here are unrecoverable from
+    // the UI they break: too small is an invisible icon, too large is a toolbar
+    // taller than the window, and in either case the control the user would
+    // reach for to fix it is the one that just broke.
+    const QVariant iconSize = settings.value(QStringLiteral("toolbar_icon_size"));
+    if (iconSize.isValid()) {
+        bool ok = false;
+        const int value = iconSize.toString().trimmed().toInt(&ok);
+        if (!ok) {
+            addProblem(QStringLiteral("Toolbar icon size '%1' is not a number; "
+                                      "using %2.")
+                           .arg(iconSize.toString())
+                           .arg(m_toolbarIconSize));
+        } else if (value < kMinToolbarIconSize || value > kMaxToolbarIconSize) {
+            m_toolbarIconSize =
+                qBound(kMinToolbarIconSize, value, kMaxToolbarIconSize);
+            addProblem(QStringLiteral("Toolbar icon size %1 is outside %2 to "
+                                      "%3; using %4.")
+                           .arg(value)
+                           .arg(kMinToolbarIconSize)
+                           .arg(kMaxToolbarIconSize)
+                           .arg(m_toolbarIconSize));
+        } else {
+            m_toolbarIconSize = value;
+        }
+    }
+
     // Absent is silent, the default being 2000. Present but unparseable warns,
     // for the same reason message_zoom does: the user asked for something and
     // is not getting it.
diff --git a/src/config.h b/src/config.h
index 3174a03..3dd9011 100644
--- a/src/config.h
+++ b/src/config.h
@@ -90,6 +90,18 @@ public:
     /// Empty when unset; the caller disables the Sync button in that case.
     QString syncCommand() const { return m_syncCommand; }
 
+    /// Toolbar icon size in pixels, 16 to 64, defaulting to 24.
+    ///
+    /// The desktop's own PM_ToolBarIconSize was the obvious default and was
+    /// rejected empirically: it reports 16 here, which is a small target now
+    /// that the toolbar follows the platform's "icon only" style and the icon
+    /// is the whole control. Setting this to 16 restores the theme's value.
+    ///
+    /// Clamped rather than trusted: a 4px icon is invisible and a 4000px one
+    /// makes the toolbar taller than the window, and neither is recoverable
+    /// from the UI the value just broke.
+    int toolbarIconSize() const { return m_toolbarIconSize; }
+
     /// The sync script's log file, read to learn the outcome of a sync this
     /// process did not start (item 54).
     ///
@@ -171,6 +183,7 @@ private:
     QList<SavedQuery> m_savedQueries;
     QString m_syncCommand;
     QString m_syncLog;
+    int m_toolbarIconSize = 24;
     QString m_notmuchConfig;
     qreal m_messageZoom = 1.0;
     bool m_completionOnFocus = false;
diff --git a/src/mainwindow.cpp b/src/mainwindow.cpp
index 865c691..a176d71 100644
--- a/src/mainwindow.cpp
+++ b/src/mainwindow.cpp
@@ -988,6 +988,14 @@ void MainWindow::buildMenus()
     // which would ignore the setting just as thoroughly in the other direction.
     toolBar->setToolButtonStyle(static_cast<Qt::ToolButtonStyle>(
         style()->styleHint(QStyle::SH_ToolButtonStyle, nullptr, toolBar)));
+
+    // Set explicitly rather than left to the style. With the button style above
+    // resolving to icon-only on this desktop, the icon IS the control, and this
+    // style's PM_ToolBarIconSize is 16px, which is a small target for it.
+    // Configurable because the right answer depends on the display, not on
+    // anything this code can see.
+    const int iconSize = m_config.toolbarIconSize();
+    toolBar->setIconSize(QSize(iconSize, iconSize));
     QAction *syncAction = m_actions.value(QStringLiteral("sync"));
     // Carried over from the QPushButton this replaced: with no command
     // configured the control is disabled, and the tooltip is the only thing