practice UI: the dock, the transcript cards, layered-Esc discard

- PracticeDock (components/practice-dock.tsx): the active problem
  replaces the composer — statement (markdown+LaTeX) above, the 'Your
  thinking' composer-twin below with the two submit lanes (Nudge =
  thinking without a commit; Commit = answer commit; ⌘/Ctrl+Enter
  commits). Dictation claims the thinking field.
- Transcript cards (practice-cards.tsx + the artifact registry): the
  problem card (compact-expandable statement — the commitment NEVER
  renders), the conclude card (outcome + verdict + reveal; the
  dismissal synthesis is one muted line), and the user-side thinking
  register cards (Your thinking / Your solution).
- view.ts: the problem pose joins the placeholder fork (the ack never
  flips the part); practice-thinking markers project as data parts;
  problem-dismissed markers project nothing.
- chat.tsx: activeProblemOf drives the dock branch (question > problem
  > composer), the esc layering gains the problem rung (below question,
  above turn) with its discard confirm + the silent marker route; the
  thinking submissions POST through the session's optimistic appends.

components/abort-turn-dialog.tsx

@@ -24,7 +24,7 @@ import { createPortal } from "react-dom";
  */
 
 /** What Esc is aborting — drives copy and the confirm action. */
-export type AbortMode = "turn" | "question";
+export type AbortMode = "turn" | "question" | "problem";
 
 const COPY: Record<AbortMode, { title: string; line: string; action: string }> = {
 	turn: {
@@ -37,6 +37,11 @@ const COPY: Record<AbortMode, { title: string; line: string; action: string }> =
 		line: "Skipped — the teacher moves on without it.",
 		action: "Dismiss",
 	},
+	problem: {
+		title: "Discard this problem?",
+		line: "The problem is dropped — no solution is revealed.",
+		action: "Discard",
+	},
 };
 
 /** The dialog's pure body — exported for SSR pins (the delete-node