Web GUI: no in-browser preview for .docx — "Not a text file, preview is unavailable for now"

[bennyfarsaci]

Summary
Clicking Open on a deliverable card (or an inline-code file reference) for a .docx file opens the right Sidebar preview and shows:

Not a text file, preview is unavailable for now.

The file itself is served correctly — there is simply no Office-document renderer in the browser half of the document-preview package. This affects every .docx deliverable; reproduced across multiple sessions.

Reproduction
In any session, have the agent create and present a .docx file (e.g., generated with python-docx).
On the closing turn's deliverables row, click the card / left side of its Open control.
The Sidebar preview opens and displays: "Not a text file, preview is unavailable for now."
The same file opened from disk (or via the chevron menu → Open with default application) is intact — confirming this is a rendering gap, not a transfer or corruption issue.

Root cause (verified in 0.1.5-rc.3)
Server side is fine. @deepseek-ai/dsh-api-session-controller mounts GET|HEAD /api/file?path= and uses the mime-types package, so .docx responses carry application/vnd.openxmlformats-officedocument.wordprocessingml.document. Bytes arrive complete.
Client side has no Office renderer. @deepseek-ai/dsh-client-ui-sidebar-documentpreview registers renderers for text/code, Markdown, HTML (sandboxed Blob iframe), PDF (pdf.js), and images only. A .docx falls through to the plain-text path and hits the error.notText locale string ("Not a text file, preview is unavailable for now." / "非文本文件，暂时无法预览。").
The only working path today is the card's chevron menu → Open with default application (native open via @deepseek-ai/dsh-native-command).
Suggested fix
Add an Office-document renderer to dsh-client-ui-sidebar-documentpreview, registered alongside the existing PDF and image renderers:

Phase 1 — .docx: convert docx → HTML in-browser with mammoth (MIT, single-file browser bundle, no native dependencies), then render through the package's existing document body component. Register it for MIME application/vnd.openxmlformats-officedocument.wordprocessingml.document and the .docx extension so the Open with dropdown offers it as a first-class viewer instead of falling back to plain text.
Follow the existing renderer contract: shared loading indicator, per-renderer locale keys (en/zh), and graceful failure strings for oversized or malformed files.
Phase 2 — optional: .xlsx / .pptx via SheetJS or equivalent, if demand warrants.
Environment
DSH 0.1.5-rc.3 (npx @deepseek-ai/dsh, Web profile)
Windows host; Web GUI at
http://127.0.0.1:3080/
Affects all .docx deliverables regardless of size or origin