XL landscape: revert tray to landscape style; fix sig-stage stretch

- Remove _tray.scss XL (≥1800px) portrait override block entirely
- _isLandscape() no longer returns false at ≥1800px — tray uses
  landscape slide-from-top at all wide landscape widths
- sig-stage: align-self: stretch (was center) so JS sizeSigCard()
  measures correct stage width; card size no longer collapses
- Position strip: horizontal row at top (was vertical column-reverse)
- sig-overlay/sig-stage/sig-deck-grid layout polish at 1100px/1800px

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

src/static_src/scss/_applets.scss

@@ -234,6 +234,16 @@
         black 99%,
         transparent 100%
     );
+    margin-left: 1rem;
+    margin-top: 1rem;
+    @media (orientation: landscape) and (min-width: 900px) {
+        margin-left: 2rem;
+        margin-top: 2rem;
+    }
+    @media (orientation: landscape) and (min-width: 1800px) {
+        margin-left: 4rem;
+        margin-top: 4rem;
+    }
 
     section {
         @extend %applet-box;