Replace hand-rolled spatial canvas drag with Konva

The custom pointer-capture + frozen-camera drag math was fragile and
felt worse than before. Konva handles the drag gesture itself; the
"me" node's screen position stays pinned to the stage center via
dragBoundFunc + a worldLayer offset that recenters on every dragmove,
so the world pans under the avatar instead of the avatar drifting off
center. Grid, peers, and the audible-range circle all live in the same
world-space layer so they pan together.

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

public/index.html

@@ -72,7 +72,7 @@
         <span>SPATIAL AUDIO</span>
         <span class="hint">drag <b>your</b> avatar &mdash; others hear you shift; you hear them relative to position</span>
       </div>
-      <canvas id="spatial"></canvas>
+      <div id="spatial"></div>
     </div>
   </main>
 </div>