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/_game-kit.scss

@@ -9,6 +9,10 @@
         top: auto;
     }
 
+    @media (orientation: landscape) and (min-width: 1800px) {
+        right: 2.5rem;  // centre in doubled 8rem sidebar
+    }
+
     z-index: 318;
     font-size: 1.75rem;
     cursor: pointer;