fix(frontend): avoid subtask render state mutation

[Gujiassh]

Summary

Fixes #3147.

This removes subtask state writes from the MessageList render path:

• computes rendered subtask snapshots from grouped messages with useMemo
• syncs those snapshots to the subtask context from useEffect
• updates subtask context through functional immutable state updates instead of mutating the existing tasks object
• keeps SubtaskCard renderable on the first frame with a fallback task snapshot before context sync completes

Validation

From frontend/:

• pnpm install --frozen-lockfile
• pnpm exec prettier --check src/core/tasks/context.tsx src/components/workspace/messages/message-list.tsx src/components/workspace/messages/subtask-card.tsx tests/unit/core/tasks/context.test.ts
• pnpm exec vitest run tests/unit/core/tasks/context.test.ts tests/unit/core/tasks/subtask-result.test.ts
• pnpm exec eslint src/core/tasks/context.tsx src/components/workspace/messages/message-list.tsx src/components/workspace/messages/subtask-card.tsx tests/unit/core/tasks/context.test.ts
• pnpm exec tsc --noEmit
• git diff --check