web: stop double-JSON-encoding the signal payload

Browser was JSON.stringify-ing the inner payload before putting it
in the outer message envelope. Go side then saw a JSON string where
it expected a JSON object, and SignalPayload Unmarshal silently
failed — which is why answers never made it back through the
peer connection.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/web/index.html

@@ -164,7 +164,7 @@
         type: "signal",
         from: peerID,
         source: "web",
-        signal: JSON.stringify(payload),
+        signal: payload,
       }),
     });
   }
@@ -216,8 +216,8 @@
       try {
         const m = JSON.parse(ev.data);
         if (m.from === peerID) return; // ignore our own
-        let payload = m.signal;
-        if (typeof payload === "string") payload = JSON.parse(payload);
+        const payload = m.signal;
+        if (!payload) return;
         if (payload.kind === "offer" && payload.sdp) {
           handleOffer(payload.sdp.sdp);
         } else if (payload.kind === "ice" && payload.candidate) {