feat: improve document management usability for issue 781

[luoye520ww]

Summary

Follow-up fixes for the issue KunAgent#781 document-management usability implementation.

This draft PR is based on the fork's develop branch, which mirrors KunAgent/Kun:develop, so review diff context matches the upstream target.

Changes

• Broaden assistant-message file reference detection, including Chinese file names and generated/code-block output.
• Remove the unsafe cross-workspace auto-preview restore shim. The renderer enhancement no longer reopens stale paths after arbitrary thread/workspace switches.
• Keep the large reading mode attached to the live preview DOM instead of cloning .ds-code-sidebar, so React-bound interactions are not turned into a dead snapshot.
• Move file-tree modified sorting into ChatFileTreePanel React state instead of writing style.order on non-flex block rows.
• Populate WorkspaceEntry.mtimeMs / WorkspaceEntry.size through a metadata-backed workspace directory listing, so modified sorting has real filesystem data.
• Define Recent files as recently modified workspace files and scan the workspace recursively, instead of only using already-expanded file-tree directories.
• Add native file-tree/recent-file drag data using the existing composer @path mention token formatting.
• Scope preview tab pinned/scroll state by page/workspace context instead of only tab label to reduce cross-workspace state collisions.
• Add focused sorting helper tests for ChatFileTreePanel.

Scope hygiene

The PR diff is intentionally limited to KunAgent#781 document-management files. Skill/runtime/locale behavior changes are not included.

Manual checks

• Assistant output paths like docs/前处理算法优化计划_v2.0.md and scripts/infer.py are clickable.
• Code-block output containing file paths can open the right-side preview.
• Switching to a different workspace no longer restores the old preview path into the new workspace.
• File tree modified sorting uses mtimeMs from the main process.
• Recent files are derived from a recursive workspace scan and sorted by modification time.
• File tree rows and recent-file rows set composer-compatible drag data.
• Click 阅读; it expands the live preview panel rather than cloning a static copy.

Validation to run locally

npm run lint
npm run typecheck
npm test

I could not run local commands from this environment; this remains draft until those pass locally.