feat(ui): act on the message a row displays, not its whole thread

A thread's card has rendered one message since item 66, but every tag
action still acted on the entire conversation. Delete, Archive,
Important, Mark spam and Toggle unread now act on the message the card
shows; the whole-thread versions move to a "Whole thread" submenu in the
Message menu and the thread list's context menu, on Ctrl+Alt+<key>.

Closes items 87, 88, 105, 106, 107, 108, 109, 110 and 111.

The defects fixed along the way, several found by reading rather than by
report:

- threadAt(current.row()) answered about the wrong thread for a reply
  row, because a tree numbers rows per parent. The audit found four live
  sites, not the one reported: Delete and Toggle unread each chose their
  DIRECTION from an unrelated thread, and the tag dialog counted the
  wrong thread's tags. threadFor(index) replaces them.

- A message-scoped write made no optimistic model update and no reply
  row carried a doomed cue, so acting on a reply moved the pending-edit
  count and changed nothing on screen.

- Both toggles read the state of a reply's THREAD, which a
  message-scoped write never changes, so they were one-way: the second
  press re-sent a tag the message already had.

- flushHeldEdits() re-sent only thread-scoped edits, so a tag change
  made on one message during a sync was applied to the row, counted as
  unsynced, and then dropped without ever being written.

- applyTagChange() updated a thread's summary but not its loaded
  replies, leaving an expanded thread's rows describing a state the
  database no longer held.

- A thread's first message is not among its children, so both
  message-scoped lookups missed it: acting on a root card repainted
  nothing and emptied the message pane's chip row.

- ThreadSummary::tags is notmuch's union over the thread, so a card
  standing for one message drew tags belonging to its siblings. The
  worker now reads that message's own tags in the walk that already
  finds its id, so the split is known before a row is ever opened.

The card shows both tiers: its own message's tags at full size, the rest
of the conversation's smaller and muted, so nothing appears to vanish
when a row is selected.

Auto mark-read is message-scoped as a result, and now arms for a reply,
which it never did. With maildir.synchronize_flags on, the old
thread-wide write reached the server for mail that had never been
displayed.

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

1 files changed, 18 insertions, 0 deletions

diff --git a/src/carddelegate.h b/src/carddelegate.h
index 74dee8e..1846359 100644
--- a/src/carddelegate.h
+++ b/src/carddelegate.h
@@ -76,4 +76,22 @@ public:
     ///
     /// Falls back to threadLineColour() for a thread with no account tag.
     static QColor accentLineColour(const QColor &accountColour);
+
+    /// A tag chip's colour, drained for the SIBLING tier (item 111).
+    ///
+    /// Saturation only: hue stays so the tag is still recognisable, and
+    /// lightness stays so `TagColors::textColourOn()` keeps choosing the same
+    /// text colour and the chip cannot become unreadable. Exposed for a test,
+    /// since "muted" has to be asserted on rather than eyeballed.
+    static QColor mutedChipColour(const QColor &chipColour);
+
+    /// The size of one tag chip on a card, for either tier.
+    ///
+    /// The delegate's own arithmetic rather than a duplicate of it: a test
+    /// calling `TagChip::sizeFor` directly proves what that function does and
+    /// nothing about what the delegate ASKS for, which is where the padding
+    /// scale is chosen. A mutation dropping the scale at the call site
+    /// survived exactly that kind of test.
+    static QSize chipSize(const QFontMetrics &metrics, const QString &text,
+                          bool own);
 };