fix(file-preview): let rendered markdown fill the resized window

The corner grip and Expand size the window correctly, but the document
kept a fixed 960px reading column from before the resize feature
existed, so text stopped reflowing once the window grew past it.
Pinned by new rule 13 in file-preview-window.test.ts.

Co-Authored-By: Claude Code <noreply@anthropic.com>

src/web/public/styles.css

@@ -12544,13 +12544,17 @@ html:is([data-skin="paper-gray"], [data-skin="solarized-light"], [data-skin="cat
 
 /* ---- File Viewer rendered markdown ----
    Styling comes from the Response Viewer's .rv-text rules; .rv-text itself
-   carries no padding or base font (the chat card supplies those). */
+   carries no padding or base font (the chat card supplies those).
+
+   No width cap. The window is whatever the user drags or expands it to, so the
+   document must reflow to fill it; the fixed 960px reading column this rule
+   shipped before the resize grip existed stops reflow once the window is wider
+   than the cap. Pinned by test/file-preview-window.test.ts. */
 
 .file-preview-body > .file-preview-md {
   padding: 1rem 1.25rem 2rem;
   font-size: 15px;
   line-height: 1.55;
-  max-width: 960px;
 }
 
 /* Relative links in the document are rebased onto a.rv-path so the Response