feat(ui): show each thread's tags under its row

The thread list was uniform and cramped: every row one line tall, with
nothing to say what a thread was about before opening it. Rows are now
roughly double height, carrying a strip of tag chips beneath the text,
with alternating row colours and a star column for flagged threads
beside the existing paperclip.

The strip is painted by the VIEW rather than by a delegate, which is
why ThreadListView exists. A delegate is handed one cell's rectangle
and cannot paint outside its column, so a strip drawn from the subject
column stops at that column's edge, losing the last tags of a
well-tagged thread, and starts at its left edge, putting the chips
under the subject instead of under the row.

Tags the row already shows another way are left out: inbox as
structure, unread as the dimming, flagged as the star, attachment as
the paperclip, and the account as the chip in the subject cell. Sorted,
since notmuch's order is not guaranteed stable and a row whose chips
reordered between repaints would flicker.

Six defects were introduced and fixed on the way here, all of them one
consequence: a QTableView paints per cell, and a row-wide strip is not
a cell. SubjectDelegate installed view-wide drew the account chip into
every column, since AccountLabelRole belongs to the row; it is split
into RowStyleDelegate for every column and SubjectDelegate for the
subject alone, with a Q_ASSERT guarding that. Row height returned from
sizeHint did nothing, because a table takes one height per row. The
strip painted from x=0 over the marker columns, via a protected
viewportMargins() that returns 0. Measuring the text band and the strip
with one font put the pills over the date. Alternating colours and the
selection are per-cell too, so the band showed bare viewport background
until the view filled it, honouring the model's own BackgroundRole
first so a deleted row is not cut in half. And that fill spanned the
full width, cutting the centred marker glyphs at their midpoint.

Closes item 5.

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

1 files changed, 14 insertions, 6 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 096eb19..1c1e87b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,15 @@ point at which they are stable.
 
 ### Added
 
+- **The thread list shows each thread's tags**, as small coloured chips in a
+  strip under the row, using the same colours as the message pane. Rows are
+  taller to make room, and alternate in colour so one can be followed across
+  the width. The strip spans the whole row rather than sitting inside the
+  subject column, so a well-tagged thread does not lose its last tags off the
+  edge. Tags the row already shows another way are left out: the account, the
+  flag, the attachment, and read state.
+- **A star column for flagged threads**, beside the existing attachment
+  paperclip.
 - **Mark all read**, on the toolbar, the Message menu and `Ctrl+Shift+U`. It
   acts on every thread in the current view rather than the selection, as one
   write and one undo entry, so a single `Ctrl+Z` puts back a view of 400
@@ -30,12 +39,11 @@ point at which they are stable.
   until it exits, then a single summary line, so a run of over a minute was
   silent and there was nothing for the status bar to report. This is not a
   buffering problem and `stdbuf` does not help.
-- **Read threads are dimmed in the list, rather than unread being bold.** Bold
-  was the only thing distinguishing the two, and on some systems it renders
-  identically to regular, which is a Qt or fontconfig matter this application
-  cannot reach: read and unread mail looked exactly alike. The distinction now
-  rides on colour, with unread left at the palette's own text colour and read
-  receding toward the background. Bold is still applied where it works.
+- **Read threads are dimmed in the list**, so unread mail stands out by colour
+  as well as by weight. Bold alone was the only distinction, which leaves
+  nothing to see when the desktop's own font is configured bold. Unread keeps
+  the palette's text colour and read recedes toward the background; bold still
+  applies on top.
 
 ### Fixed