fix(settings-prompt): widen the Diff dialog (max-w-7xl never applied on desktop)

The Diff dialog passed a bare `max-w-7xl`, but DialogContent's default already
sets `sm:max-w-lg`. tailwind-merge keeps both (different variants), and at ≥sm the
`sm:` rule wins the cascade — so the dialog was pinned at 512px and the split diff
overflowed with a horizontal scrollbar. Use `sm:max-w-7xl` so it overrides the
default at the same breakpoint, matching the app's other wide dialogs. Verified
live: the real Diff dialog now renders at 1280px with no horizontal scroll.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/pages/settings/settings-prompt.tsx

@@ -997,7 +997,7 @@ function SettingsPrompt() {
                 onOpenChange={setIsDiffDialogOpen}
                 open={isDiffDialogOpen}
             >
-                <DialogContent className="max-w-7xl">
+                <DialogContent className="sm:max-w-7xl">
                     <DialogHeader>
                         <DialogTitle className="flex items-center gap-2">
                             <FileDiff className="size-5" />