docs: specify the thread pane as a card list

Item 53 recorded that message rows read as a table and left the approach
unspecified, with four directions ranging from spanning columns on reply rows to
abandoning message rows entirely. The user's answer is wider than all four: the
column grid is wrong for the WHOLE left pane, threads included.

Threads and replies both become cards in a single column, three lines each, at
one uniform height. Sender and date, then the subject with the flag, attachment
and reply-count marks inline, then the tag chips. Replies indent by depth with a
continuous spine, capped at depth 4.

Three decisions worth their reasoning, since each closed an option that looked
cheaper:

  - Uniform height keeps setUniformRowHeights(true), which is the single
    cheapest property of the design. A blank third line under untagged cards
    buys constant sizeHint arithmetic everywhere else.
  - Uncapped indent with a horizontally scrollable pane was asked for and
    rejected: it reopens item 51 in a worse form. Cards are viewport width, so
    the pane has no horizontal scroll range at all, and item 51 is resolved for
    free rather than fought.
  - A reply's line 3 shows only the tags its thread does not have. The full
    per-message set was rejected on measurement, not taste: of 48691 messages in
    the user's database, 7 carry unread and 75 carry flagged, and both are
    already shown as the sender's weight and the mark on line 2. Everything else
    is applied per thread and identical on every message in it, so full sets
    would render blank on nearly every reply and identical chips on the rest.

The design is a net removal. ThreadListView::paintEvent, the tag strip's band
arithmetic, SubjectDelegate, RowStyleDelegate, the five Column enumerators and
the decorative header all go; one CardDelegate paints the whole card. That
retires the two bug classes CLAUDE.md documents for the strip, a deleted row cut
in half and every other row showing a bare stripe, both of which existed because
the strip spanned cells it did not own.

The column header was decorative, so a sort dropdown adds a feature rather than
replacing one. Two entries only, newest and oldest, passed to notmuch. Sorting
by sender or subject would have to happen in the model after results arrive,
which fights the batching that makes a 10k-thread query paint immediately.

Item 51 is marked resolved by 53 rather than left as separate work.

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