feat(config): let the date format on a card be configured

Adds [general] date_format, a QDateTime pattern for the date a thread card
shows. Absent or empty means the system locale's short format, which is what
every other application on the desktop uses and stays the default.

The format reaches the LAYOUT, not only the painter. CardLayout::compute()
reserves the date's width from widestDateSample(), so a pattern that arrived
only at the drawText call would be elided into a rect sized for the old
format, which is the clipping the bold-font fault already produced once. It
rides on CardLayout::Input and defaults to an empty string, leaving every
existing call site unchanged. Confirmed by mutation: making the width ignore
the format fails the test.

widestDateSample() memoised its result in a static, which would have sized
every format after the first from whichever arrived first. It is a plain call
now, costing one QLocale lookup per row, the same as formatting the date.

Validation rejects only a pattern whose output is CONSTANT, found by
formatting two different instants and comparing. QDateTime::toString() treats
nearly every letter as a field, so "banana" formats as "bpmnpmnpm" and
"hello" as "22ello": nonsense, but they vary with the instant, and a check
claiming to find "no date field" cannot reject them. What harms the user is
the pattern that prints the same text on every card, and that is what is
refused, with the value named in the message.

The model supplies the pattern through DateFormatRole for the same reason it
supplies the tag colours: it is the one object here holding config, and a
delegate reading config itself would be a second source of truth.

Backlog item 62.

1 files changed, 46 insertions, 0 deletions

diff --git a/tests/test_cardlayout.cpp b/tests/test_cardlayout.cpp
index fdc18bf..bce6a0f 100644
--- a/tests/test_cardlayout.cpp
+++ b/tests/test_cardlayout.cpp
@@ -40,6 +40,7 @@ private slots:
     void replyCardCarriesNoAccentBar();
     void theDateFitsWhenTheCardIsBold();
     void theDateFollowsTheSystemLocale();
+    void aConfiguredDateFormatIsUsedAndReservedFor();
 };
 
 namespace {
@@ -343,6 +344,51 @@ void TestCardLayout::theDateFollowsTheSystemLocale()
              "formatting of a date");
 }
 
+void TestCardLayout::aConfiguredDateFormatIsUsedAndReservedFor()
+{
+    const QDateTime when(QDate(2025, 8, 10), QTime(6, 26));
+
+    // A pattern deliberately much longer than any locale's short format, so
+    // the width claim below cannot pass by accident on a locale whose own
+    // dates happen to be wide enough already.
+    const QString format = QStringLiteral("dddd d MMMM yyyy 'at' hh:mm:ss");
+
+    QCOMPARE(CardLayout::formatDate(when, format),
+             QLocale::system().toString(when, format));
+    QVERIFY2(CardLayout::formatDate(when, format)
+                 != CardLayout::formatDate(when),
+             "a configured format produced the same string as the system one, "
+             "so the parameter is being ignored");
+
+    // An empty format is what an absent or rejected config key gives, and it
+    // must mean the system format rather than an empty date.
+    QCOMPARE(CardLayout::formatDate(when, QString()),
+             CardLayout::formatDate(when));
+
+    // The load-bearing half: the reserved width has to follow the SAME format,
+    // or a long pattern is elided into a rect sized for a short one. This is
+    // the fault that a static, format-independent widest-date sample produces.
+    QFont font;
+    const int h = CardLayout::heightFor(font);
+    CardLayout::Input in = threadInput();
+    in.dateFormat = format;
+    const CardLayout card = CardLayout::compute(in, QRect(0, 0, 900, h), font);
+    QFont bold = font;
+    bold.setBold(true);
+    QVERIFY2(card.dateRect.width()
+                 >= QFontMetrics(bold).horizontalAdvance(
+                        CardLayout::formatDate(when, format)),
+             "the reserved date width is narrower than the configured format's "
+             "own output");
+
+    // And it is genuinely wider than the default's, which proves the width
+    // moved with the format rather than a generous constant covering both.
+    const CardLayout plain =
+        CardLayout::compute(threadInput(), QRect(0, 0, 900, h), font);
+    QVERIFY2(card.dateRect.width() > plain.dateRect.width(),
+             "a longer date format reserved no more width than the default");
+}
+
 void TestCardLayout::theDateFitsWhenTheCardIsBold()
 {
     // An UNREAD card draws BOLD, and bold is wider. The layout is computed from