feat: integrate art assets across all game panels (#43)

## Summary

- Adds `apps/web/src/utils/cdn.ts` with a `cdnImage(folder, id)` helper that builds URLs from `https://cdn.nhcarrigan.com/elysium/`
- Wires CDN art into all 13 game panels (bosses, quests, adventurers, companions, equipment, upgrades, prestige, transcendence, achievements, explorations, crafting, story, codex)
- Zone selector tabs now display 16:9 zone art thumbnails in place of emoji icons
- Adds a fixed background image at 15% opacity via `body::before`
- Documents the art generation and CDN upload process in `CLAUDE.md` for future expansions

Resolves #15

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #43
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

apps/web/src/components/game/zoneSelector.tsx

@@ -4,6 +4,7 @@
  * @license Naomi's Public License
  * @author Naomi Carrigan
  */
+import { cdnImage } from "../../utils/cdn.js";
 import type { Zone } from "@elysium/types";
 import type { JSX } from "react";
 
@@ -44,7 +45,11 @@ const ZoneSelector = ({
             title={zone.description}
             type="button"
           >
-            <span className="zone-emoji">{zone.emoji}</span>
+            <img
+              alt={zone.name}
+              className="zone-tab-image"
+              src={cdnImage("zones", zone.id)}
+            />
             <span className="zone-name">{zone.name}</span>
           </button>
         );