LED wall active area: confine the output to a pixel-mapping rectangle

New 'Area LED' controls in the Schermo tab: W x H @ X,Y. Everything
(canvas, videos, images, logos, ticker) lives in a #stage box whose
transform makes it the containing block for the fixed layers, so resizing
it confines the whole show at once, black outside. The canvas resizes to
the stage, so shaders render at the wall's native resolution. Persisted
in localStorage and re-pushed when the output window (re)starts; the
recorder follows the canvas position.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/output.html

@@ -8,6 +8,8 @@
   <link rel="stylesheet" href="style.css" />
 </head>
 <body class="output">
+  <!-- LED-wall active area: every layer lives inside; black outside -->
+  <div id="stage">
   <canvas id="gl"></canvas>
 
   <!-- Video layer (VJ loop) that blends with the generated visuals -->
@@ -39,6 +41,7 @@
     <div class="big">🎛 In attesa dell'audio…</div>
     <div class="small">Usa il pannello di controllo sull'altro schermo</div>
   </div>
+  </div><!-- /#stage -->
 
   <script src="shaders.js"></script>
   <script src="effects.js"></script>