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, 4 insertions, 0 deletions

diff --git a/src/threadlistmodel.cpp b/src/threadlistmodel.cpp
index 21a6378..33356f0 100644
--- a/src/threadlistmodel.cpp
+++ b/src/threadlistmodel.cpp
@@ -323,6 +323,8 @@ QVariant ThreadListModel::data(const QModelIndex &index, int role) const
             // A reply never offers an expander: nesting past the first level is
             // drawn from depth, not from further parent-child structure.
             return 0;
+        case DateFormatRole:
+            return m_dateFormat;
         case Qt::BackgroundRole:
             // Tinted, so an expanded thread reads as one block rather than as
             // more table rows. Applied per cell here; ThreadListView fills the
@@ -480,6 +482,8 @@ QVariant ThreadListModel::data(const QModelIndex &index, int role) const
     case ReplyCountRole:
         // totalCount includes the root message, which is the card itself.
         return qMax(0, thread.totalCount - 1);
+    case DateFormatRole:
+        return m_dateFormat;
     default:
         break;
     }