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, 16 insertions, 2 deletions

diff --git a/src/cardlayout.h b/src/cardlayout.h
index ef6a563..1a2fd18 100644
--- a/src/cardlayout.h
+++ b/src/cardlayout.h
@@ -46,6 +46,16 @@ struct CardLayout
         bool isMessage = false;
         int depth = 0;       ///< 0 for a thread root, 1 for a direct reply.
         int replyCount = 0;  ///< 0 means no expander.
+
+        /// A QDateTime::toString() pattern from [general] date_format, or empty
+        /// for the system's short format.
+        ///
+        /// It lives on the INPUT rather than being read where the date is
+        /// drawn, because the width reserved for the date is computed from the
+        /// same format inside compute(). A pattern reaching the painter but not
+        /// the geometry is exactly how a longer date gets elided into a rect
+        /// sized for a shorter one.
+        QString dateFormat;
     };
 
     /// Width of the account accent bar down a thread card's left edge.
@@ -129,10 +139,14 @@ struct CardLayout
     /// drawn into it come from one place: a locale whose short format is
     /// longer than the reserved rect would clip, which is exactly the fault
     /// bold text produced.
-    static QString formatDate(const QDateTime &date);
+    /// `format` is a QDateTime::toString() pattern, or empty for the system's
+    /// short format. Config validates it, so an unusable pattern never gets
+    /// this far.
+    static QString formatDate(const QDateTime &date,
+                              const QString &format = QString());
 
     /// The widest string formatDate() can return, for reserving space.
-    static QString widestDateSample();
+    static QString widestDateSample(const QString &format = QString());
 
     /// The expander's label: the reply count with its glyph, as drawn.
     ///