XL landscape breakpoint (≥1800px): double sidebar widths + scale content

- _base.scss: new @media (orientation:landscape) and (min-width:1800px) block —
  sidebars 4rem→8rem; navbar btn 3rem→5rem; brand h1 1.2rem→2.4rem; navbar-text
  0.65rem→1.3rem; footer icons 1.75rem→3rem; nav gap 3rem→4rem; footer-container
  0.55rem→0.85rem; container margins 4rem→8rem; h2 portrait-style (2rem, centred)
- _applets.scss: gear btn right 0.5rem→2.5rem; menus right 0.5rem→2rem at ≥1800px
- _game-kit.scss: kit btn right 0.5rem→2.5rem at ≥1800px
- _room.scss: sig-overlay padding-left 4rem→8rem at ≥1800px
- _tray.scss: tray wrap left/right 4rem→8rem at ≥1800px
- room.js: sizeSigModal right inset 64px→128px at ≥1800px viewport width

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/static_src/scss/_base.scss

@@ -34,6 +34,7 @@ body {
                     font-size: 2rem;
                 }
             }
+
             
             .container-fluid {
                 display: flex;
@@ -358,6 +359,58 @@ body {
     }
 }
 
+// ── XL landscape (≥1800px): double sidebar widths and scale content ────────────
+@media (orientation: landscape) and (min-width: 1800px) {
+    $sidebar-xl: 8rem;
+
+    body .container .navbar {
+        width: $sidebar-xl;
+
+        .container-fluid {
+            gap: 2rem;
+            padding: 0 0.5rem;
+        }
+
+        .navbar-brand h1  { font-size: 2.4rem; }
+        .navbar-text      { font-size: 1.3rem; }
+
+        .btn-primary {
+            width: 5rem;
+            height: 5rem;
+            font-size: 1.125rem;
+        }
+
+        .input-group {
+            left:  $sidebar-xl;
+            right: $sidebar-xl;
+        }
+    }
+
+    body .container {
+        margin-left:  $sidebar-xl;
+        margin-right: $sidebar-xl;
+    }
+
+    // h2 page title: portrait-style — centred and full-size on a wide canvas
+    body .container .row .col-lg-6 h2 {
+        font-size: 2rem;
+        letter-spacing: 0.33em;
+        text-align: center;
+        text-align-last: center;
+    }
+
+    body #id_footer {
+        width: $sidebar-xl;
+
+        #id_footer_nav {
+            gap: 4rem;
+            a { font-size: 3rem; }
+        }
+
+        .footer-container { font-size: 0.85rem; }
+    }
+}
+
 @media (orientation: portrait) and (max-width: 500px) {
     body .container  {
         .navbar {