The stats bar: context + cost beneath the composer, per-turn detail

One quiet row beneath the composer/question shell (not flush to the
screen bottom; static height, outside the morphing shell):

- LEFT: context window + session cost, clickable → per-turn detail
  popover (context, total, unpriced count, one row per turn with
  model/steps/cost). All folds from core/prices.ts — sessionCost,
  contextWindow, turnCost — no cost math in components.
- RIGHT: model + "Thinking —" — a placeholder slot for the coming
  model/thinking-level CONTROL (later work with its contract changes);
  display-only today, model read from the last turn's usage,
  pretty-printed (Glm 5.3 Flash).
- Data: the record's turns log, seeded on mount, refreshed when the
  local stream settles, kept fresh by the existing record re-syncs.
  No turns yet → "—" everywhere; unpriceable turns never fold into a
  fake $0 (the doctrine).
- formatUsd/formatTokens moved to components/usage-format.ts (shared
  with the activity dock); prettyModel joins them, pure + tested.

tests/render-activity-dock.test.tsx

@@ -1,7 +1,8 @@
 import { renderToStaticMarkup } from "react-dom/server";
 import { describe, it } from "node:test";
 import assert from "node:assert/strict";
-import { ActivityDock, formatElapsed, formatTokens, formatUsd, toBlobs, type ActivityFeed } from "../components/activity-dock.tsx";
+import { ActivityDock, formatElapsed, toBlobs, type ActivityFeed } from "../components/activity-dock.tsx";
+import { formatTokens, formatUsd } from "../components/usage-format.ts";
 
 // The dock's pure layer: folds, formatting, and blob order — then the SSR
 // render (blobs, "—" doctrine, the abort affordance, the narrow badge).