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/question-dock.tsx

@@ -853,7 +853,7 @@ function QuestionDockForm({
 									submit();
 								}
 							}}
-							placeholder="Why this answer? What are you unsure about? Say as much as you like — the teacher reads all of this."
+							placeholder="Why this answer? Whatever comes to mind."
 							rows={1}
 							className={cn(
 								"field-sizing-content lh-input max-h-48 w-full resize-none bg-transparent px-2.5 py-2 outline-none placeholder:text-muted-foreground/70",