feat: relay cross-machine vital/debuff sharing for MosswartMassacre

Accepts new share_subscribe / share_unsubscribe / share_* WebSocket
messages from MM plugin clients and fans them out to other opted-in
plugin clients (excluding origin) and to browser clients for the
NetworkUI window.

- main.py: _vital_sharing_subscribers set, _vital_sharing_peer_state
  snapshot, _broadcast_share_to_plugin_clients relay, disconnect
  cleanup, GET /vital-sharing/peers endpoint.
- static/index.html: new sidebar link for Vital Sharing window.
- static/script.js: showVitalSharingWindow with live HP/STA/MANA bars,
  per-peer status dot/tags/position, 5s /vital-sharing/peers poll, and
  share_* routing through the existing browser WebSocket handler.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/index.html

@@ -104,6 +104,13 @@
         📋 Issues Board
       </a>
     </div>
+
+    <!-- Vital Sharing Network link -->
+    <div class="quest-status-link">
+      <a href="#" id="vitalSharingBtn" onclick="showVitalSharingWindow()">
+        🤝 Vital Sharing
+      </a>
+    </div>
     
     <!-- Container for sort and filter controls -->
     <div id="sortButtons" class="sort-buttons"></div>