feat: add GitHub Actions CI and support hosting under a subpath

Mirrors the GitLab pipeline — typecheck, tests, feed sanity — and adds a
Pages deploy.

Pages serves from /<repo>/, and the app used absolute asset paths, so that
job would have shipped a site that 404s on everything. Assets now resolve
against a <base href> the build substitutes, the feed URL resolves against
document.baseURI so deep links work too, and the service worker derives its
paths from its own registration scope. Root-hosted builds are unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

src/client/api.ts

@@ -13,7 +13,10 @@ export type FeedState =
  * worse than one that asks you to reload.
  */
 export async function fetchFeed(signal?: AbortSignal): Promise<EventFeed> {
-  const res = await fetch("/data/events.v1.json", { signal: signal ?? null });
+  // Resolved against <base>, so this is correct at a domain root, under a
+  // GitHub Pages subpath, and on a deep link alike.
+  const feedUrl = new URL("data/events.v1.json", document.baseURI);
+  const res = await fetch(feedUrl, { signal: signal ?? null });
   if (!res.ok) {
     throw new Error(`Feed request failed (${res.status}).`);
   }

src/client/main.tsx

@@ -15,7 +15,8 @@ createRoot(root).render(
 // guarded because file:// and older browsers have no service worker at all.
 if ("serviceWorker" in navigator && location.protocol.startsWith("http")) {
   window.addEventListener("load", () => {
-    void navigator.serviceWorker.register("/sw.js").catch(() => {
+    const swUrl = new URL("sw.js", document.baseURI);
+    void navigator.serviceWorker.register(swUrl).catch(() => {
       // An unavailable worker costs offline support, nothing else. The app
       // works exactly as before.
     });

src/client/manifest.webmanifest

@@ -2,15 +2,15 @@
   "name": "Event Clock",
   "short_name": "Event Clock",
   "description": "Live and upcoming gacha events, sorted by what expires next.",
-  "start_url": "/",
-  "scope": "/",
+  "start_url": "./",
+  "scope": "./",
   "display": "standalone",
   "background_color": "#12141c",
   "theme_color": "#12141c",
   "orientation": "portrait-primary",
   "icons": [
     {
-      "src": "/icon.svg",
+      "src": "icon.svg",
       "sizes": "any",
       "type": "image/svg+xml",
       "purpose": "any maskable"

src/client/sw.js

@@ -15,8 +15,12 @@
  */
 
 const CACHE_VERSION = "event-clock-v1";
-const SHELL = ["/", "/index.html", "/styles.css", "/main.js"];
-const FEED = "/data/events.v1.json";
+// Paths are derived from the registration scope, so the same worker is
+// correct at a domain root and under a subpath (GitHub Pages) alike.
+const BASE = new URL("./", self.registration.scope);
+const at = (path) => new URL(path, BASE).toString();
+const SHELL = ["", "index.html", "styles.css", "main.js"].map(at);
+const FEED = new URL("data/events.v1.json", BASE).pathname;
 const FONT_HOSTS = new Set(["fonts.googleapis.com", "fonts.gstatic.com"]);
 
 self.addEventListener("install", (event) => {