feat(frontend): preview text-capture entries (.md / .txt)

Text captures land as `.md` (Markdown) or `.txt` (plain) blobs, but
PreviewPanel only dispatched on video/audio/image/pdf/html extensions,
so opening a text entry hit the 'No preview available' fallback with
the raw artifact path exposed.

- New `TextPreview` component fetches the primary artifact as text,
  renders it in a monospace `<pre>` with word-wrap, and shows the
  entry title on top and the MIME as a small trailing tag. Handles
  loading/error states.
- `PreviewPanel` gains a `TEXT_EXTS` set + a branch that dispatches
  to `TextPreview` for `md` / `markdown` / `txt`.
- CSS is padded and centered to ~780px so a text note reads like a
  document rather than an edge-to-edge terminal dump.

v1 intentionally does NOT parse Markdown: keeping frontend deps at
react+react-dom only. Bump to a real Markdown renderer if we start
capturing Markdown-authored notes.

frontend/src/components/PreviewPanel.jsx

@@ -2,10 +2,12 @@ import VideoPreview from './VideoPreview';
 import IframePreview from './IframePreview';
 import ImagePreview from './ImagePreview';
 import TweetPreview from './TweetPreview';
+import TextPreview from './TextPreview';
 
 const VIDEO_EXTS = new Set(['mp4', 'webm', 'mov', 'mkv', 'avi', 'm4v', 'ogv']);
 const AUDIO_EXTS = new Set(['mp3', 'ogg', 'm4a', 'opus', 'wav', 'flac', 'aac']);
 const IMAGE_EXTS = new Set(['jpg', 'jpeg', 'png', 'gif', 'webp', 'avif', 'svg', 'bmp']);
+const TEXT_EXTS  = new Set(['md', 'markdown', 'txt']);
 const CONTENT_TYPES = {
   mp4: 'video/mp4', webm: 'video/webm', mov: 'video/quicktime',
   mkv: 'video/x-matroska', avi: 'video/x-msvideo', m4v: 'video/mp4', ogv: 'video/ogg',
@@ -131,7 +133,20 @@ export default function PreviewPanel({ archiveId, entry, detail, fullPage, onXAr
     );
   }
 
-  // 7. Image
+  // 6b. Plain text / Markdown
+  if (TEXT_EXTS.has(ext)) {
+    return (
+      <div className="preview-panel" style={{ flex: 1, minHeight: 0, overflow: 'auto' }}>
+        <TextPreview
+          src={primaryMediaUrl}
+          mime={primaryArtifact.mime_type}
+          title={summary.title}
+        />
+      </div>
+    );
+  }
+
+    // 7. Image
   if (IMAGE_EXTS.has(ext)) {
     return (
       <div className="preview-panel" style={{ height: '100%' }}>

frontend/src/components/TextPreview.jsx

@@ -0,0 +1,46 @@
+import { useEffect, useState } from 'react'
+
+/**
+ * Renders the primary_media artifact of a text/document entry as plain text.
+ *
+ * v1 intentionally does NOT render Markdown — we don't want a Markdown parser
+ * dep just to unblock the "no preview available" fallback, and monospace text
+ * with visible fences reads fine for the note-length content this feature
+ * captures. Bump to a real renderer if/when Markdown-authored entries grow.
+ */
+export default function TextPreview({ src, mime, title }) {
+  const [text, setText] = useState(null)
+  const [error, setError] = useState(null)
+
+  useEffect(() => {
+    let cancelled = false
+    setText(null)
+    setError(null)
+    fetch(src, { credentials: 'same-origin' })
+      .then(r => {
+        if (!r.ok) throw new Error(`HTTP ${r.status}`)
+        return r.text()
+      })
+      .then(t => { if (!cancelled) setText(t) })
+      .catch(e => { if (!cancelled) setError(e.message || String(e)) })
+    return () => { cancelled = true }
+  }, [src])
+
+  if (error) {
+    return (
+      <div className="text-preview text-preview--error">
+        Failed to load text: {error}
+      </div>
+    )
+  }
+  if (text === null) {
+    return <div className="text-preview text-preview--loading">Loading…</div>
+  }
+  return (
+    <div className="text-preview">
+      {title && <h1 className="text-preview__title">{title}</h1>}
+      <pre className="text-preview__body">{text}</pre>
+      {mime && <div className="text-preview__mime">{mime}</div>}
+    </div>
+  )
+}