fix(ui): expand flat threads, reach the first message, localise the date

Four faults from the first hand test, two of them behavioural.

An expander that opened onto nothing. setThreadMessages kept only nodes with
depth > 0, and notmuch_thread_get_toplevel_messages returns every message at
depth 0 when a thread carries no usable In-Reply-To, so a flat thread
contributed no children while its card still advertised the count. Measured in
the user's database: of 396 inbox threads three are flat, one of them nine
messages long, and every two-message thread of that kind was affected, which is
exactly why the fault looked like "the expander only works with more than one
reply". The rule is now position, not depth: every message except the first,
which is the root card itself. That is also the correct rule rather than a
workaround, since the row under the root is the second message however notmuch
chose to nest it.

The thread's first message was unreachable. Selecting a root card loaded the
whole thread, so the pane showed every message with only the last expanded, and
no row in the list offered the first one: the reply rows are messages two
onward. The root card now renders its own message, which is what the card
already claims to be. It keeps its thread id, unlike the message-row path, so
mark-read and the tag-change repaint still work; that is asserted, because
clearing it is the obvious way to write this and silently disables both. Before
the replies are loaded the model has no first message to name and the whole
thread stays the honest answer.

Dates ignored the locale. One hardcoded "yyyy-MM-dd hh:mm" produced a US-looking
format on an Italian desktop; QLocale::system() now formats it, and the width
reserved for the date comes from the same function so a longer locale cannot
clip.

The expander was a bare number on the card's own background. It is a pill now,
carrying "3 replies" (and "1 reply", singular), sized from the label actually
drawn and measured in both glyph states so it does not resize under the pointer
on click. Its fill is blended from Text toward Base rather than taken from
QPalette::Button, which is #2b2b2b against a Base of #2b2b2b on the user's
theme: byte identical, so the pill was invisible. A theme may make any two roles
equal; a blend is defined against the surface it sits on and cannot collide with
it. Checked by rendering both a dark and a light palette and looking.

1 files changed, 53 insertions, 5 deletions

diff --git a/src/cardlayout.cpp b/src/cardlayout.cpp
index 1a79e3b..0e118ab 100644
--- a/src/cardlayout.cpp
+++ b/src/cardlayout.cpp
@@ -19,6 +19,44 @@
 #include "cardlayout.h"
 
 #include <QFontMetrics>
+#include <QLocale>
+
+QString CardLayout::formatDate(const QDateTime &date)
+{
+    // The system locale's own short format, not a hardcoded pattern: an
+    // Italian desktop writes 10/08/2025, not 2025-08-10, and a mail client
+    // that disagrees with every other application on screen is simply wrong.
+    return QLocale::system().toString(date, QLocale::ShortFormat);
+}
+
+QString CardLayout::expanderLabel(int replyCount, bool expanded)
+{
+    // "3 replies", not a bare "3". The count alone reads as an unexplained
+    // number beside the subject, and the word is what says the card opens.
+    //
+    // Not translated through tr() here because CardLayout is a plain struct
+    // rather than a QObject; the delegate is where a translated build would
+    // wrap this, and the string is deliberately kept in one place so there is
+    // exactly one thing to change.
+    const QString glyph = expanded ? QStringLiteral("\u25be")
+                                   : QStringLiteral("\u25b8");
+    const QString word = replyCount == 1 ? QStringLiteral("reply")
+                                         : QStringLiteral("replies");
+    return QStringLiteral("%1 %2 %3").arg(glyph).arg(replyCount).arg(word);
+}
+
+QString CardLayout::widestDateSample()
+{
+    // A real date run through the same formatter, with the wide digits and a
+    // two-digit day and month, so the reserved width matches what is drawn
+    // whatever the locale's pattern turns out to be. Guessing a pattern here
+    // would reintroduce the clipping this exists to prevent.
+    static const QString sample = [] {
+        const QDateTime wide(QDate(2028, 12, 28), QTime(22, 58));
+        return formatDate(wide);
+    }();
+    return sample;
+}
 
 QFont CardLayout::smallFont(const QFont &cardFont)
 {
@@ -96,8 +134,8 @@ CardLayout CardLayout::compute(const Input &input, const QRect &rect,
     // painted.
     QFont dateFont = font;
     dateFont.setBold(true);
-    const int dateWidth = QFontMetrics(dateFont).horizontalAdvance(
-        QStringLiteral("8888-88-88 88:88"));
+    const int dateWidth =
+        QFontMetrics(dateFont).horizontalAdvance(widestDateSample());
     out.dateRect = QRect(right - dateWidth, lineOneTop, dateWidth,
                          metrics.height());
     out.senderRect = QRect(out.contentLeft, lineOneTop,
@@ -105,10 +143,20 @@ CardLayout CardLayout::compute(const Input &input, const QRect &rect,
                                        - kPaddingX),
                            metrics.height());
 
-    // The expander is the reply count, on line two and on the right.
+    // The expander is the reply count as a PILL, on line two and on the right.
+    //
+    // Sized from the label actually drawn rather than from a fixed sample, so
+    // the background and the text inside it cannot disagree. Both states of the
+    // glyph are measured because the rect must not change width when the card
+    // is expanded: a pill that resized on click would shift the subject's
+    // elision under the pointer.
     if (input.replyCount > 0) {
-        const int countWidth = smallMetrics.horizontalAdvance(
-            QStringLiteral("▾ 8888 replies"));
+        const int collapsed = smallMetrics.horizontalAdvance(
+            expanderLabel(input.replyCount, false));
+        const int expanded = smallMetrics.horizontalAdvance(
+            expanderLabel(input.replyCount, true));
+        const int countWidth =
+            qMax(collapsed, expanded) + kPillPaddingX * 2;
         out.expanderRect = QRect(right - countWidth, lineTwoTop, countWidth,
                                  metrics.height());
     }