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/storyPanel.tsx

@@ -9,6 +9,7 @@
 import { STORY_CHAPTERS } from "@elysium/types";
 import { type JSX, useState } from "react";
 import { useGame } from "../../context/gameContext.js";
+import { cdnImage } from "../../utils/cdn.js";
 
 /**
  * Substitutes the character name placeholder in story text.
@@ -102,6 +103,11 @@ const StoryPanel = (): JSX.Element => {
         : <div className="story-chapter-view">
           {isUnlocked
             ? <>
+              <img
+                alt={activeChapter.title}
+                className="story-chapter-banner"
+                src={cdnImage("story-chapters", activeChapter.id)}
+              />
               <h2 className="story-chapter-title">
                 {"Chapter "}
                 {activeChapterIndex + 1}