fix(files): an HTML file in the Changes panel shows as the page only on Preview

Changes stays the default view and Whole file shows the source, as for any
file. A Preview button — offered on the Changes diff too, where it was
missing — opens the page; Raw, Changes or Whole file go back to the source.

docs/LANES.md

@@ -678,8 +678,9 @@ reads the workspace's git surface in five modes:
   least one line tall (`MIN_ROW`), so an empty filler row keeps the two
   columns aligned. An image file (png, jpg, gif, webp, avif, bmp, ico, svg) is
   shown as the picture (`/git/raw`), an HTML file (`.html`/`.htm`) opens
-  as the page itself — **Raw** shows its source in the editor, **Preview**
-  brings the page back — and a conflicted file opens in the
+  like any file (Changes, or Whole file as source) with a **Preview**
+  button — on Changes too — that shows it as the page; **Raw** goes back to
+  its source — and a conflicted file opens in the
   conflict view. The page (here, in a Chat file preview, and on a
   double-click in the File Explorer, whose right-click menu keeps
   **Download**) runs in the chat artifact's sandbox (`HtmlFrame.tsx`: