[Feature]: Files panel: add a toggle to show dot-prefixed files and folders

## Summary

Add a user-controlled toggle to show dot-prefixed files and folders in the Files side panel. Hidden by default is reasonable, but there is currently no way to reveal these entries from the UI.

## 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

The Files panel always hides files and directories whose name starts with a dot. Everyday workspace files become unmanageable through the panel: .gitignore, .env templates, the .mcp directory, or dot files that agents themselves create and expect the user to review.

The listing endpoint `GET /tree` skips every entry whose name starts with a dot, see `list_directory` in `src/qwenpaw/services/workspace_files.py`. The endpoint exposes no `show_hidden` parameter and the frontend offers no toggle. The filter affects listings only. The read and write endpoints resolve dot-prefixed paths normally, so the files are accessible but cannot be seen or opened from the panel.

Anyone managing git metadata, environment templates, or MCP configuration in an agent workspace benefits from this.

## Proposed Solution

Either form would work: a per-session toggle in the Files panel that resets on reload, or a persistent setting, global or per workspace, that defaults to off. When enabled, the listing includes entries whose names start with a dot.

## Alternatives Considered

- Browse the workspace folder directly in a file manager or editor.
- Ask the agent to edit the file in chat, since agent tools bypass the panel filter.
- Create a symlink with a non-dot name pointing to a frequently needed file.

These work for single files but do not scale to browsing or reviewing dot directories.

## Additional Context

Tested on version 2.2.0.

## Willing to Contribute

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