refactor(canvas): decouple the canvas from the chat engine

The canvas components will be reused by the MCP server's browser shell,
which has no chat and no Next.js. They now read everything they need
from small contexts and the canvas store instead of the chat engine:

- components/canvas/versions-context.tsx: VersionsProvider and
  useVersionsContext give the version cards, the strip and the compare
  dialog a VersionsSource (versions, onCanvasId, undoneId, isBusy,
  canUndo, canRedo, restore, undo, redo). The chat fills it from the
  versions store and the engine in components/chat/chat-versions.tsx.
- stores/canvas-store.ts: isBusy and busyReason; the chat engine sets
  them while a turn runs, SelectionAsk and the version UI read them.
- components/canvas/version-card.tsx: the visual version card and the
  thumbnail, out of tool-activity.tsx; the chat's card composes it and
  adds its "Show XML" link and code panel.
- compare-dialog.tsx and version-strip.tsx move to components/canvas;
  the strip takes the minimum number of versions to show as a prop, the
  chat panel computes it from the cards it has.
- components/canvas/locale-context.tsx: LocaleProvider and useLocale,
  fed by the [lang] layout; CanvasStage no longer uses next/navigation.
  The overlay (SelectionAsk) is a slot and the wait for the saved
  language is a prop, so the shell can leave both out.
- lib/version-text.ts: describeChanges is now describeChangeSummary, so
  it can be imported next to the MCP core's describeChanges.

lib/version-text.ts

@@ -17,7 +17,7 @@ const PARTS: [keyof ChangeSummary, string][] = [
  * singular (...One) and plural (...Other) form; languages without plurals
  * use the same text for both.
  */
-export function describeChanges(
+export function describeChangeSummary(
     summary: ChangeSummary,
     t: VersionDict,
 ): string {