copy: the thinking-field placeholders, the brief round (release polish 5)

Four placeholders rewritten per the owner's set (2026-10-07): the
thinking fields are an invitation to stream-of-consciousness transfer,
and everything else is implied by the field's existence — no "the
teacher reads all of this", no mechanism, no em dashes, nothing that
wraps. The reading field's placeholder no longer breaks to a second
line.

  quiz:     "Why this answer? Whatever comes to mind."
  reading:  "Whatever comes to mind as you read."
  practice: "Work here, think out loud."
  pause:    "Ask anything, or press Enter for the next one."

The answer-field placeholders ("Type your answer…") stay: brief and
functional already (owner ruling). Quiz placeholder pin updated.

1428 tests, tsc, lint, build green.

components/chat.tsx

@@ -1574,7 +1574,7 @@ export function Chat({
 							// applies.
 							nextProblem={nextReady}
 							onNext={sendNext}
-							placeholder={nextReady ? "Ask about this problem — or press Enter for the next one…" : undefined}
+							placeholder={nextReady ? "Ask anything, or press Enter for the next one." : undefined}
 						/>
 					}
 					onSubmitAnswer={(output) => {