[Feature]: "Edit last message" and "Rewind" button in the Web UI

## Summary

Add native **Edit-last-message** and **Rewind** controls to the web UI so users can correct their own previous prompt or roll back to an earlier turn without restarting the whole session.

## Component(s) Affected

- [x] Core / Backend (app, agents, config, providers, utils, local_models)
- [x] Console (frontend web UI)
- [ ] Channels (DingTalk, Feishu, QQ, Discord, iMessage, etc.)
- [ ] Skills
- [ ] CLI
- [ ] Documentation (website)
- [ ] Tests
- [ ] CI/CD
- [ ] Scripts / Deploy

## Problem / Motivation

Once a user sends a message, there is no built-in way to:

- **Edit their own last message** — if the prompt was worded poorly or a detail is missing, the user must either retype the whole thing in a new turn or start over. There is no in-place correction.
- **Rewind** — roll back the conversation to a previous turn and drop everything after it, so a different path can be taken.

These are standard in modern chat UIs (ChatGPT, Claude) and materially improve UX during experimentation and iterative refinement. Today the only workaround is manual re-prompting, which pollutes the prompt history.

## Proposed Solution

Introduce buttons in the web ui next to the "copy message" button:

1. **Edit last message** — An "Edit" action on the most recent *user* message that rewrites it **in place** and re-sends it to the model. The last LLM response is regenerated.
2. **Rewind** — A button that rolls the session back to a selected turn, discarding all messages after that point. Show a confirmation dialog to prevent accidental data loss.

## Alternatives Considered

- **Manual re-prompting** — re-typing the corrected prompt in a new turn. Simple, but pollutes the history and less intuitive when the user scrolls back in the web ui

## Additional Context

- Follow-up to [#4703](https://github.com/agentscope-ai/QwenPaw/issues/4703) ("Native fork/rewind/regen for conversations")
A plugin has been created by the author but it is outdated and command driven only.

## Willing to Contribute

- [ ] I am willing to open a PR for this feature (after discussion).

The issue identifies the Core/Backend and Console frontend as affected, but names no files, tests, or entry points. Start by tracing the existing conversation/session handling across those components and review the related design in issue #4703. Done should include clearly defined edit and rewind behavior, confirmation for discarded turns, regenerated responses, and coverage for the new UI flows.

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