practice polish: no phantom wake, Practice header tag, honest meta

- needsContinuation: a persisted problem-pose ack (terminate:true ended
  the turn deliberately) owes no continuation — reloads after a pose no
  longer wake a phantom turn; error results stay owed like any
  interrupted tool.
- Chat header: practice sessions swap the phase chip for a quiet
  Practice tag (no probe→teach arc applies) and hide the coverage ring
  (practice never extracts — an ever-filling ring would lie).
- ChatSummary gains pendingProblem (activeProblemOf at projection); the
  sidebar meta says 'Problem waiting' for a practice row with a posed
  problem. Kind threads through Chat/[id] page.
- Home picker chip gains Clear (picks were otherwise irremovable).

src/core/contracts/chat.ts

@@ -233,6 +233,9 @@ export interface ChatSummary {
 	phase: Phase3;
 	/** An unanswered quiz/ask question is waiting for the learner. */
 	pendingQuestion: boolean;
+	/** Practice sessions (2026-10-04): a posed problem with no conclude —
+	 *  the sidebar meta's "Problem waiting". Absent on teaching sessions. */
+	pendingProblem: boolean;
 	/** The record's model-control settings — rides the list payload via the
 	 *  summary spread (the picker never needs it from here; the full record
 	 *  is its source). */