feat(v2): 13 improvements — functional, visual, UX, backend

Functional:
1. Chat: "▼ New messages below" indicator when scrolled up, click to jump
2. Combat stats: "Clear Session" button (red, with confirm dialog)
3. Inventory: live updates via inventory_delta WS (re-fetches on change)
4. Inventory: real mana time from equipment_cantrip_state WS (live
   countdown with state dot: green=active, red=inactive, yellow=unknown)

Visual:
5. Thin separator line between tool links and sort buttons
6. Selected player row highlighted with darker background (#2a3344)
7. Scroll-to-top button (▲) appears when scrolled past 200px in player list

UX:
8. Double-click player dot on map opens their chat window
9. Right-click player dot shows context menu (Chat/Stats/Inv/Char/Combat/Radar)
10. Ctrl+D keyboard shortcut toggles between map and dashboard views
11. Sound notification on rare drops (880Hz sine beep via Web Audio API)

Backend:
12. Deep-merge lifetime offense/defense per element — accumulates
    total_attacks, failed_attacks, crits, damage per AttackType×Element
    instead of overwriting with latest session data
13. Startup cleanup: deletes stale combat_stats records from before
    the lifetime fix (pre-2026-04-14T09:00Z)

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

frontend/src/components/windows/CombatStatsWindow.tsx

@@ -100,10 +100,17 @@ export const CombatStatsWindow: React.FC<Props> = ({ id, charName, zIndex }) =>
 
   return (
     <DraggableWindow id={id} title={`Combat: ${charName}`} zIndex={zIndex} width={640} height={520}>
-      {/* Toggle */}
-      <div style={{ display: 'flex', gap: 4, padding: '4px 8px', borderBottom: '1px solid #333' }}>
+      {/* Toggle + Clear */}
+      <div style={{ display: 'flex', gap: 4, padding: '4px 8px', borderBottom: '1px solid #333', alignItems: 'center' }}>
         <button className={`ml-stats-range-btn ${mode === 'session' ? 'active' : ''}`} onClick={() => setMode('session')}>Session</button>
         <button className={`ml-stats-range-btn ${mode === 'lifetime' ? 'active' : ''}`} onClick={() => setMode('lifetime')}>Lifetime</button>
+        <div style={{ flex: 1 }} />
+        {mode === 'session' && (
+          <button style={{ fontSize: '0.6rem', padding: '2px 8px', background: 'rgba(204,68,68,0.15)', color: '#c66', border: '1px solid rgba(204,68,68,0.3)', borderRadius: 3, cursor: 'pointer' }}
+            onClick={() => { if (confirm('Clear current session stats?')) { /* Send clear command via socket if available, or just clear local */ setData((d: any) => d ? { ...d, session: { total_damage_given: 0, total_damage_received: 0, total_kills: 0, total_aetheria_surges: 0, total_cloak_surges: 0, monsters: {} } } : d); } }}>
+            Clear Session
+          </button>
+        )}
       </div>
       <div style={{ display: 'flex', flex: 1, overflow: 'hidden' }}>
         {/* Monster list (left) */}