fix/rendering issues for text based files on chat preview and toggle for .md files

[AlanAAG]

Summary

• Clicking a generated .md (or any URL-backed text) attachment in chat entered a perpetual loading state, with the Network tab showing an endless cascade of aborted/retried requests.
• Adds a Preview/Source toggle for markdown attachments so users can view rendered markdown or raw text.

Root cause

AttachmentPreviewModal's kind = classify(attachment) returned a new object on every render instead of being memoized. The text-loading useEffect depended on kind, so its own setTextLoading/setTextContent calls triggered a re-render, which produced a new kind reference, which re-fired the effect — aborting and re-issuing the same fetch in an infinite loop. This wasn't

Summary

• Clicking a generated .md (or any URL-backed text) attachment in chat entered a perpetual loading state, with the Network tab showing an endless cascade of aborted/retried requests.
• Adds a Preview/Source toggle for markdown attachments so users can view rendered markdown or raw text.

Root cause

AttachmentPreviewModal's kind = classify(attachment) returned a new object on every render instead of being memoized. The text-loading useEffect depended on kind, so its own setTextLoading/setTextContent calls triggered a re-render, which produced a new kind reference, which re-fired the effect — aborting and re-issuing the same fetch in an infinite loop. This wasn't .md-specific; it affected any URL-backed text-classified attachment (agent-generated attachments never carry inline base64 content).

Fix: memoize kind with useMemo(() => ..., [attachment]), matching the pattern already used for imageSrc/pdfBlobUrl in the same file.

Feature

• classify() now also flags isMarkdown (.md files only).
• Markdown attachments default to a rendered Preview (reusing the existing MarkdownContent/react-markdown pipeline used in chat bubbles), with a Source tab to see raw text.
• Toggle only appears for .md; other text types (.json, .csv, etc.) are unchanged.

Test plan

• npx tsc --noEmit — no new errors introduced (pre-existing CSS-module resolution gap in this project is unrelated, confirmed present before this change too)
• Manually verify in the app: generate a .md file via chat, click to open — preview loads once (no repeated network requests), toggle to Source and back works, non-.md text attachments still preview as raw text.

Closes #369