Tracking B3b: change cards webview UI and rollback buttons

[easonLiangWorldedtech]

Tracking issue for the B3b PR (branch feat/change-cards-webview-b3b; PR to be opened after review).

Part of the file-write safety series — plan: easonLiangWorldedtech/Zoo-Code#33, epic: #1375. Item: B3b — Change cards webview UI + rollback buttons. Stacks on the B3a PR (#1401); the card payloads and rollback actions already exist on the extension host.

Scope

• Render the per-step change card in the checkpoint chat message: the summary line ("N files changed this step") + per-file rows; when changeCardDetail === "full" the per-file unified diff is expandable.
• Rollback buttons: per-file ("restore this file") and per-step ("restore this step"), posting the B3a checkpointRestoreFile / checkpointRestoreStep messages, with a confirm step and the existing error surfacing on failure.
• The changeCardDetail select control in the Checkpoints settings section (the round trip already exists from B3a) with i18n labels in every locale.

Acceptance

• Card renders for summary-only payloads (no diff rows) and for full payloads (expandable diffs); suppressed checkpoints still render in history
• Rollback buttons post the exact B3a payloads; a failed rollback surfaces the B3a i18n error string
• Settings select binds to cachedState (not live state) and saves via handleSubmit
• Behavioral webview tests (Vitest JSDOM) for render + button dispatch; CI green (ubuntu unit-test gate), 100% codecov patch coverage on the webview-ui flag

Status

• PR: to be opened (after B3a review/ship)

PR: #1412