feat: add last-turn diff view

Adds a Last turn scope to DiffView that renders OpenCode snapshot diffs from the latest user message summary without re-fetching git contents. The view hides Review in that mode and carries the selected diff scope through main and context-panel navigation.

Connects latest-turn changed-file chips in chat to the snapshot diff view on desktop and mobile, while keeping older turn chips static/read-only to avoid misleading affordances and extra subscriptions. Updates localized labels and empty states plus changelog.

Validation: bun run type-check (packages/ui); bun run lint (packages/ui).

packages/ui/src/components/chat/lib/turns/types.ts

@@ -115,6 +115,7 @@ export interface TurnGroupingContext {
     activityOwnerMessageId?: string;
     isFirstAssistantInTurn: boolean;
     isLastAssistantInTurn: boolean;
+    isLatestTurn: boolean;
     summaryBody?: string;
     activityParts?: TurnActivityRecord[];
     activityGroupSegments?: TurnActivityGroup[];