Preserve chat history after summarization

[LittleChenLiya]

Summary

• archive messages that are about to be removed by summarization into display_messages
• hide generated summary messages from the chat UI while keeping them in the model context
• render archived display messages before the current compressed message state
• add e2e coverage for the summarized-history display behavior

Root cause

Long-running ultra-mode chats can trigger summarization. The middleware replaces older messages with a summary message for model context management, and the UI renders that same compressed state, so users lose visibility into their original inputs and earlier conversation turns.

Validation

• cd backend && uv run pytest tests\test_summarization_middleware.py -q
• cd backend && uvx ruff check .
• cd backend && uvx ruff format --check .
• cd frontend && pnpm.cmd format
• cd frontend && pnpm.cmd lint
• cd frontend && pnpm.cmd typecheck
• cd frontend && BETTER_AUTH_SECRET=local-dev-secret pnpm.cmd build
• cd frontend && pnpm.cmd exec playwright test tests/e2e/chat.spec.ts --config=playwright.edge.local.config.ts --grep "shows archived messages" (local Edge config only, removed after run)