feat: combat stats backend + frontend (Mag-Tools style)

Backend:
- db_async.py: new combat_stats + combat_stats_sessions tables
- main.py: combat_stats message handler with DB upsert (lifetime +
  session snapshots), in-memory live_combat_stats dict, broadcast
  to browser clients.
- REST: GET /combat-stats and GET /combat-stats/{character_name}

Frontend:
- index.html: new "Combat Stats" sidebar link
- script.js: full Combat Stats window with two panels:
  - Top: monster list (name, kills, dmg recv, dmg given) with
    clickable rows and "All" aggregate, matching CombatTrackerGUI.cs
  - Bottom: damage breakdown grid matching CombatTrackerGUIInfo.cs
    layout — element × attack type matrix (Mel/Msl + Magic columns),
    Attacks (hit%), Evades (%), Resists (%), A.Surges (%), C.Surges (%),
    normal Avg/Max, Crits (%), Crit Avg/Max, Total Damage.
  - Session / Lifetime toggle button
- style.css: combat-stats-toggle styles

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

static/index.html

@@ -111,6 +111,13 @@
         🤝 Vital Sharing
       </a>
     </div>
+
+    <!-- Combat Stats link -->
+    <div class="quest-status-link">
+      <a href="#" id="combatStatsBtn" onclick="showCombatStatsWindow()">
+        Combat Stats
+      </a>
+    </div>
     
     <!-- Container for sort and filter controls -->
     <div id="sortButtons" class="sort-buttons"></div>