Add workspace binding to channel settings for project file access

[edpipole]

Description

Add workspace binding support to channel settings so that conversations created by IM channels (Telegram, Lark, DingTalk, WeChat) can have access to project files.

Currently, when a message arrives from an IM channel, the backend creates a conversation with an empty workspace (workspace=''), resulting in a temporary directory with no access to project files.

Proposed Changes

1. New API Endpoint

PUT /api/channel/settings/{platform}/workspace
Body: { "workspace": "/path/to/project" }

This should persist the workspace path in channel settings, similar to the existing PUT /api/channel/settings/{platform}/assistant and PUT /api/channel/settings/{platform}/default-model endpoints.

2. Update IChannelPlatformSettings Response

The GET /api/channel/settings/{platform} response should include the workspace field:

{
  "platform": "telegram",
  "assistant": { "assistant_id": "..." },
  "default_model": { "id": "...", "use_model": "..." },
  "workspace": "/path/to/project"
}

3. Use Workspace When Creating Channel Conversations

When a message arrives from an IM channel and the backend creates/reactivates a conversation:

• If settings.workspace is non-empty, use it as the conversation's workspace (equivalent to custom_workspace: true)
• If settings.workspace is empty/null, maintain current behavior (temporary workspace)

This gives the assistant access to project files when responding to IM messages.

Context

• Frontend changes (types, IPC route, UI) have been implemented in the AionUi desktop client on branch feat/channel-workspace-binding
• The IChannelSession type already has a workspace?: string field that can be used
• The TChatConversation.extra.workspace field already exists and is used by regular conversations

Files to Update (AionCore)

• Channel settings handler — add workspace to platform settings struct
• Channel conversation creation — use settings.workspace when creating conversations from IM messages
• API routes — add PUT /api/channel/settings/{platform}/workspace