feat(phase6): Add sounds, team chat, reactions, monitor, settings, and CSV import/export

Sound System:
- Add soundStore with volume/mute persistence
- Add useSound hook with Web Audio API fallback
- Add SoundControl component for in-game volume adjustment
- Play sounds for correct/incorrect, steal, timer, victory/defeat

Team Chat:
- Add TeamChat component with collapsible panel
- Add team_message WebSocket event (team-only visibility)
- Store up to 50 messages per session

Emoji Reactions:
- Add EmojiReactions bar with 8 emojis
- Add ReactionOverlay with floating animations (Framer Motion)
- Add rate limiting (1 reaction per 3 seconds)
- Broadcast reactions to all players in room

Admin Monitor:
- Add Monitor page showing active rooms from Redis
- Display player counts, team composition, status
- Add ability to close problematic rooms

Admin Settings:
- Add Settings page for game configuration
- Configure points/times by difficulty, steal penalty, max players
- Store config in JSON file with service helpers

CSV Import/Export:
- Add export endpoint with optional filters
- Add import endpoint with validation and error reporting
- Add UI buttons and import result modal in Questions page

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/data/config.json

@@ -0,0 +1,19 @@
+{
+  "points_by_difficulty": {
+    "1": 100,
+    "2": 200,
+    "3": 300,
+    "4": 400,
+    "5": 500
+  },
+  "times_by_difficulty": {
+    "1": 15,
+    "2": 20,
+    "3": 25,
+    "4": 35,
+    "5": 45
+  },
+  "steal_penalty_percent": 50,
+  "max_players_per_team": 4,
+  "steal_time_percent": 50
+}