ui: the document look — one language for every thinking, no bubbles

Owner round 2 (2026-10-05), Slice F:

- USER TEXT: the right-aligned bubble is gone. User messages are
  full-width document rows — muted 'You' label, muted quote rail,
  full-opacity text. Learner words are content, not a chat-app tell.
- USER THINKING (everywhere): arrives COLLAPSED as a trigger row in the
  exact register of the agent's reasoning rows (muted label + chevron +
  hover affordance); expanding opens the quote block — rail, 50%
  opacity, italics. The inner-voice signature. The ThinkingBlock loses
  its shell/border: pure document material.
- QUIZ CARDS: the answer's thinking is the SAME collapsible row, same
  placement, just inside the container; the explanation drops to 50%
  opacity (upright — italics stays the thinking signature).
- AGENT'S THINKING: the duration bookkeeping is gone (it was live-turn
  state that died to 'a few seconds' on remount anyway) — the settled
  row reads static 'Agent's thinking', pairing with 'Your thinking'.
  Spinner + shimmer stay the working signal while streaming.
- Glance law: user thinking = rail + 50% italic; agent thinking = no
  rail; user text = upright, full opacity, 'You' label.

components/practice-cards.tsx

@@ -97,16 +97,16 @@ export function ConcludeCard({ part }: ArtifactPartProps) {
 }
 
 export function ThinkingCard({ data }: { data: PracticeThinkingData }) {
-	// One component, two registers (owner ruling): the committed register
-	// carries the primary rail (the quiz answer thinking's paint), the
-	// working register the muted one.
+	// The unified row (owner round, 2026-10-05): one style — collapsed
+	// arrival, 50%-italic quote block on expand. The register survives only
+	// in the label ("Your solution" when committed) and the data-commit
+	// flag for the tests.
 	return (
 		<ThinkingBlock
 			testId="thinking-card"
 			dataCommit={data.commit}
 			label={data.commit ? "Your solution" : "Your thinking"}
 			text={data.text}
-			rail={data.commit ? "primary" : "muted"}
 		/>
 	);
 }