ui: the chat comeback — side-based authorship, restored quotes, tucked whys

Blockquotes: the callout override had displaced Streamdown's own styled
blockquote component, so plain quotes rendered as bare unstyled text
since that commit. The pass-through branch now restores Streamdown's
verbatim classes (border-l-4, muted italic).

Explanations tucked for ALL questions (owner): the prediction's Why /
Hide-why toggle is universal — every quiz answered card sits its
explanation behind the same collapsed button. Same pattern, one law.

The thinking row's rhythm: my-1 -> my-3. The 4px margin hugged the
options on non-graded answered cards (owner report); 12px matches the
card's mt-2/mt-3 spacing grammar everywhere the row renders.

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 once quotes rendered. User rows go
RIGHT: 'You' messages and practice thinking cards hug the right edge at
85% width with the rail mirrored (border-r), text still reading
left-to-right. In-card thinking (inside the answered card, the agent's
context) stays left. Transcript thinking rows arrive EXPANDED — the
thinking was clearly sent. Cmd+T (toggle-user-thinking, chat scope,
cheat-sheet row) sets every user-thinking row to one state via a module
store: explicit sets stomp individual toggles, late mounts honor the
last set (the state initializer reads the store, so static render
reflects it too).

components/md-blocks.tsx

@@ -335,7 +335,21 @@ export function parseAlertMarker(children: ReactNode): ParsedAlert | null {
 export function MarkdownBlockquote(props: ComponentProps<"blockquote"> & ExtraProps) {
 	const { children, node, ...rest } = props;
 	const alert = parseAlertMarker(children);
-	if (!alert) return <blockquote {...rest}>{children}</blockquote>;
+	if (!alert) {
+		// Plain quote — Streamdown's own styling, restored by hand: overriding
+		// `blockquote` displaced its styled component (the classes below are
+		// its verbatim defaults), and bare <blockquote> rendered unstyled
+		// (owner report, 2026-10-07).
+		return (
+			<blockquote
+				{...rest}
+				className="my-4 border-l-4 border-muted-foreground/30 pl-4 text-muted-foreground italic"
+				data-streamdown="blockquote"
+			>
+				{children}
+			</blockquote>
+		);
+	}
 	return (
 		<div
 			data-testid="md-callout"