ui: round four — thinner rail, tighter band, the accent step, early wrap

Owner round four on the user rows: (1) the rail thins - the gap inside
the 3ch inset grows 5px -> 8px, so the rail is calc(3ch-8px); total
inset still 3ch, chevron law intact. (2) The band padding drops again,
5px -> 4px (py-1). (3) The in-card thinking band steps to bg-accent -
the same ONE-STEP lift the chat band gets (background->card on the page,
card->accent on the card); card-color on card-color was invisible, the
accent gives the card rows a visible band with identical heights. (4)
pr-4 on the band rows - the expanded text wraps EARLY instead of hugging
the band's clear right edge.

components/chat.tsx

@@ -1406,7 +1406,7 @@ export function Chat({
 										    inset (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-[calc(3ch-5px)] border-muted-foreground/30 bg-card py-[5px] pl-[5px] pr-2 font-ui text-xs"
+										className="my-2 w-full border-l-[calc(3ch-8px)] border-muted-foreground/30 bg-card py-1 pl-[8px] pr-4 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