feat: add about page with versions, changelog, and how-to-play

- New GET /about API endpoint caches Gitea releases for 5 minutes
- AboutPanel displays client version (via Vite define), API version, collapsible changelog, and How to Play guide
- GiteaRelease and AboutResponse types added to shared package

apps/api/src/routes/about.ts

@@ -0,0 +1,46 @@
+import { readFileSync } from "node:fs";
+import { dirname, join } from "node:path";
+import { fileURLToPath } from "node:url";
+import { Hono } from "hono";
+import type { AboutResponse, GiteaRelease } from "@elysium/types";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const { version: API_VERSION } = JSON.parse(
+  readFileSync(join(__dirname, "../../package.json"), "utf-8"),
+) as { version: string };
+
+const GITEA_RELEASES_URL =
+  "https://git.nhcarrigan.com/api/v1/repos/nhcarrigan-ideation/elysium/releases";
+const CACHE_TTL_MS = 5 * 60 * 1000;
+
+let releasesCache: GiteaRelease[] = [];
+let cacheTimestamp = 0;
+
+const fetchReleases = async (): Promise<GiteaRelease[]> => {
+  const now = Date.now();
+  if (releasesCache.length > 0 && now - cacheTimestamp < CACHE_TTL_MS) {
+    return releasesCache;
+  }
+  try {
+    const response = await fetch(GITEA_RELEASES_URL);
+    if (!response.ok) {
+      return releasesCache;
+    }
+    releasesCache = (await response.json()) as GiteaRelease[];
+    cacheTimestamp = now;
+    return releasesCache;
+  } catch {
+    return releasesCache;
+  }
+};
+
+export const aboutRouter = new Hono();
+
+aboutRouter.get("/", async (context) => {
+  const releases = await fetchReleases();
+  const body: AboutResponse = {
+    apiVersion: API_VERSION,
+    releases,
+  };
+  return context.json(body);
+});