ui: round three — the tight band, one geometry everywhere

Owner round three on the user rows: (1) the band padding halves,
py-2.5 -> py-[5px] — subtle, not tall; same for the messages. (2) The
in-card thinking joins the band: identical styling and heights for
collapsed thinking in the chat and inside question cards (card-on-card
is quiet by design; revert if it reads too light). (3) The left gap
MATCHES the new 5px padding, so the rail fills the rest of the 3ch
inset - border-l-[calc(3ch-5px)] pl-[5px]: a thick muted rail, total
inset still 3ch (the chevron-alignment law survives). The banded prop
dies - one geometry, cards included.

tests/render-practice-surface.test.tsx

@@ -185,7 +185,7 @@ describe("the practice transcript cards", () => {
 		assert.match(trace, /aria-expanded="true"/);
 		assert.ok(trace.includes("stuck at step two"), "the text is visible on arrival");
 		assert.ok(trace.includes("bg-card"), "the card-color band");
-		assert.ok(trace.includes("border-l-[6px]"), "the 6px rail, left side");
+		assert.ok(trace.includes("border-l-[calc(3ch-5px)]"), "the rail fills the 3ch inset");
 		const legacy = renderToStaticMarkup(<ThinkingCard data={{ problemCallId: "c", commit: true, text: "It is 42." }} />);
 		assert.match(legacy, /YOUR THINKING/);
 		assert.doesNotMatch(legacy, /Your solution/);