Fix /new horizontal overflow at ~800px viewports

The .deck grid used '1fr 300px'; the 1fr editor column could not shrink
below its min-content, expanding the grid to ~1000px at 800px viewports
and pushing .pane-r to right=980 (document.scrollWidth 980 vs
innerWidth 800). Use minmax(0, 1fr) so the editor column can shrink;
the editor itself already clips/scrolls its own content.

Verified with a CDP overflow probe at 800x600, 375x812, 1400x900 and
700x600: scrollWidth == innerWidth at all four.

internal/web/static/app.css

@@ -129,7 +129,7 @@ body {
 
 /* new paste page */
 .deck {
-  display: grid; grid-template-columns: 1fr 300px; gap: 16px;
+  display: grid; grid-template-columns: minmax(0, 1fr) 300px; gap: 16px;
   padding: 16px 20px 20px; height: calc(100vh - 76px);
   max-width: 1400px; margin: 0 auto;
 }