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/explorationPanel.tsx

@@ -9,6 +9,7 @@
 import { type JSX, useState } from "react";
 import { useGame } from "../../context/gameContext.js";
 import { EXPLORATION_AREAS } from "../../data/explorations.js";
+import { cdnImage } from "../../utils/cdn.js";
 import { ZoneSelector } from "./zoneSelector.js";
 import type { ExploreCollectResponse } from "@elysium/types";
 
@@ -230,6 +231,11 @@ const ExplorationPanel = (): JSX.Element => {
               className={`exploration-card exploration-${status}`}
               key={area.id}
             >
+              <img
+                alt={area.name}
+                className="card-thumbnail"
+                src={cdnImage("explorations", area.id)}
+              />
               <div className="exploration-info">
                 <h3>
                   {area.name}