ui: the document, all-in — card-color bands and the 6px rail

Owner round two on user-vs-agent clarity (the chat-comeback was tried
and set aside): we go all in on documents. No chat sides. User messages
and every transcript user-thinking row are full-width CARD-COLOR BANDS
- the band is the authorship mark against the agent's transparent
prose. No rounded corners, no borders besides the rail.

The rail keeps its muted color but is now 6px wide (was 2px): the
artificially wide 3ch inset (the chevron-alignment law with the agent's
flush rows) is re-split - rail eats the air, the rail-to-text gap drops
from 3ch-2px to a natural 3ch-6px, and the total inset stays 3ch so the
chevrons still align. The 'You' label stays on messages - one register
with the thinking labels.

In-card thinking (inside answered cards) takes the rail but NO band:
card-color on card-color would be invisible; geometry alone carries it.
Transcript rows arrive expanded (clearly sent) - the reading lift now
opens on arrival too; the in-card row stays collapsed. Cmd+T still
rules all rows.

Also this round: the answered-card last-child rhythm law
([&>*:last-child]:mb-0) - on ask cards the thinking row's my-3 stacked
with the card's p-4 into a 28px void below (12px above); the card's own
padding is the bottom rhythm when a row ends the card.

components/chat.tsx

@@ -1399,12 +1399,13 @@ export function Chat({
 										    Rendered WITHOUT the Message wrapper — that is the bubble. */
 											<div
 										data-testid="user-message"
-										/* SIDE-BASED AUTHORSHIP (owner round, 2026-10-07): the
-										    document shape blurred user into agent — three left-rail
-										    idioms (user text, user thinking, quoted prose) collided.
-										    User rows go RIGHT: mirrored rail, ~85% width, text still
-										    reading left-to-right (the chat convention). */
-										className="my-1 ml-auto w-[85%] border-r-2 border-muted-foreground/30 pr-[calc(3ch-2px)] font-ui text-xs"
+										/* THE DOCUMENT, ALL-IN (owner round two, 2026-10-07): no
+										    chat side — the user message is a full-width card-color
+										    BAND with the 6px rail, the same geometry as the thinking
+										    rows (the band is the authorship mark against the agent's
+										    transparent prose). The "You" label stays — one register
+										    with the thinking labels. */
+										className="my-2 w-full border-l-[6px] border-muted-foreground/30 bg-card py-2.5 pl-[calc(3ch-6px)] pr-2 font-ui text-xs"
 									>
 												{/* The rail GROUPS (owner round 4, 2026-10-05): the YOU heading
 												    sits INSIDE the rail, same mono-xs register and same 1ch