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/utils/cdn.ts

@@ -0,0 +1,20 @@
+/**
+ * @file CDN URL utility for Elysium game assets.
+ * @copyright nhcarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+const cdnBase = "https://cdn.nhcarrigan.com/elysium";
+
+/**
+ * Returns the CDN URL for a game asset image.
+ * @param folder - The asset category folder (e.g. "bosses", "companions").
+ * @param id - The asset identifier (file name without extension).
+ * @returns The full CDN URL for the asset.
+ */
+const cdnImage = (folder: string, id: string): string => {
+  return `${cdnBase}/${folder}/${id}.jpg`;
+};
+
+export { cdnImage };