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.

crates/archivr-server/static/index.html

@@ -6,8 +6,8 @@
     <title>Archivr</title>
     <link rel="icon" type="image/svg+xml" href="/favicon.svg">
     <link rel="icon" type="image/x-icon" href="/favicon.ico">
-    <script type="module" crossorigin src="/assets/index-CQcpPA_I.js"></script>
-    <link rel="stylesheet" crossorigin href="/assets/index-BLxoi9rt.css">
+    <script type="module" crossorigin src="/assets/index-C4rpqY7g.js"></script>
+    <link rel="stylesheet" crossorigin href="/assets/index-BV7QPxEc.css">
   </head>
   <body>
     <div id="root"></div>