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/style.css

@@ -2872,3 +2872,19 @@ table.ts-allegiance td:first-child {
 .user-info-logout:hover {
   color: #ff6b6b;
 }
+
+/* ---------- Combat Stats window ---------- */
+.combat-stats-toggle {
+  padding: 2px 10px;
+  font-size: 0.75rem;
+  background: #2a2a2a;
+  color: #aaa;
+  border: 1px solid #555;
+  cursor: pointer;
+  border-radius: 3px;
+}
+.combat-stats-toggle.active {
+  background: #3a5070;
+  color: #fff;
+  border-color: #5588bb;
+}