feat(slots): add artwork assets for dragons-hoard slot game

Add sprite images and UI elements for the dragons-hoard slot game:
- Symbol sprites (dragon, gem, knight, castle, chest)
- Cabinet and card background images
- Update slots-artwork.json with image paths for all symbols

This completes the visual asset integration for the dragons-hoard game theme.

public/data/slots-artwork.json

@@ -264,7 +264,7 @@
     {
       "id": "dragons-hoard-gem",
       "key": "slots-dragons-hoard-gem",
-      "path": null
+      "path": "/assets/images/slots/dragons-hoard/gem.png"
     },
     {
       "id": "dragons-hoard-jack",
@@ -274,7 +274,7 @@
     {
       "id": "dragons-hoard-knight",
       "key": "slots-dragons-hoard-knight",
-      "path": null
+      "path": "/assets/images/slots/dragons-hoard/knight.png"
     },
     {
       "id": "dragons-hoard-queen",
@@ -284,7 +284,7 @@
     {
       "id": "dragons-hoard-castle",
       "key": "slots-dragons-hoard-castle",
-      "path": null
+      "path": "/assets/images/slots/dragons-hoard/castle.png"
     },
     {
       "id": "dragons-hoard-ace",
@@ -294,7 +294,7 @@
     {
       "id": "dragons-hoard-chest",
       "key": "slots-dragons-hoard-chest",
-      "path": null
+      "path": "/assets/images/slots/dragons-hoard/chest.png"
     },
     {
       "id": "dragons-hoard-coin",
@@ -304,17 +304,17 @@
     {
       "id": "dragons-hoard-dragon",
       "key": "slots-dragons-hoard-dragon",
-      "path": null
+      "path": "/assets/images/slots/dragons-hoard/dragon.png"
     },
     {
       "id": "dragons-hoard-cabinet",
       "key": "slots-dragons-hoard-cabinet",
-      "path": null
+      "path": "/assets/images/slots/slots-dragons-hoard-cabinet.png"
     },
     {
       "id": "dragons-hoard-card",
       "key": "slots-dragons-hoard-card",
-      "path": null
+      "path": "/assets/images/slots/slots-dragons-hoard-card.png"
     },
     {
       "id": "gold-rush-jack",