fix(ui): tell read threads from unread without relying on bold

Bold was unread's only cue, and it renders identically to regular on the
user's system: confirmed by eye against a bare QTableView holding a
plain QStandardItemModel, with no code from this project involved. The
fault is in Qt or fontconfig, below this application, and nothing in the
model could ever have reached it. Read and unread mail looked exactly
alike.

The emphasis is inverted instead. Unread rows keep the palette's own
text colour and read rows are dimmed toward the background, so the cue
rides on Qt::ForegroundRole, which the delegate already honours, and
costs no column. It also suits the real ratio, measured at 99 unread
against 4220 read: dimming the bulk is calmer than highlighting it. The
dim colour is derived from the palette, never hardcoded, per the rule
item 12 established. Bold is kept for systems where it works, but
nothing depends on it now.

That exposed a second defect, visible the moment it shipped. Qt resolves
ForegroundRole into the palette and then prefers it over
HighlightedText, so a model-supplied colour wins on a SELECTED row too.
The dim is blended against the unselected background, so a selected read
row painted grey on the selection colour, near unreadable.
SubjectDelegate::initStyleOption now reverses that, and the delegate is
installed view-wide rather than on the subject column alone, so every
column gets the same handling instead of three of them keeping Qt's
ordering.

The guarding tests state the property rather than the mechanism: strip
the font from the model's answer and the two states must still differ.
A test asserting only that bold is set passes on a system where bold
paints like regular, which is exactly how this survived. The selection
test renders two rows identical but for the unread tag, selects both,
and requires zero differing pixels.

Part of item 5; the density work and the star column remain.

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

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