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/companionPanel.tsx

@@ -8,6 +8,7 @@
 /* eslint-disable max-lines-per-function -- Complex companion card with conditional renders */
 import { COMPANIONS, type Companion } from "@elysium/types";
 import { useGame } from "../../context/gameContext.js";
+import { cdnImage } from "../../utils/cdn.js";
 import type { JSX } from "react";
 
 const bonusLabels: Record<string, string> = {
@@ -96,6 +97,11 @@ const CompanionCard = ({
         : ""}`}
     >
       <div className="companion-header">
+        <img
+          alt={companion.name}
+          className="card-thumbnail"
+          src={cdnImage("companions", companion.id)}
+        />
         <div className="companion-name-block">
           <span className="companion-name">{companion.name}</span>
           <span className="companion-title">{companion.title}</span>