fix(frontend): center capped content column on wide viewports (#50)

The .page column was max-width 72rem but left-aligned, leaving a large
empty band on the right above ~1200px. margin-inline: auto centers it;
narrow widths are unchanged since the column fills the viewport anyway.

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/frontend/src/index.css

@@ -1096,6 +1096,8 @@ code {
   /* ---------- page scaffold ---------- */
   .page {
     max-width: 72rem;
+    width: 100%;
+    margin-inline: auto;
     display: flex;
     flex-direction: column;
     gap: 1.5rem;