frontend: prefix-agnostic fetch paths (reverse-proxy subpath safe)

login.html/app.js now resolve fetch targets relative to the page's
directory (BASE), so the app works served at / or under a path prefix
(https://host/ani/) as long as the proxy strips the prefix.

static/app.js

@@ -1,11 +1,17 @@
 // app.js — ani-web frontend: search -> detail -> play, vanilla JS + hls.js
+// BASE = this page's directory, so the app works at / or behind a
+// prefixing reverse proxy (e.g. https://host/ani/)
+const BASE = (() => {
+  const p = location.pathname;
+  return p.endsWith("/") ? p : p.slice(0, p.lastIndexOf("/") + 1);
+})();
 const main = document.getElementById("main");
 let hls = null, detail = null, lang = "eng", seasonId = null;
 
 async function api(path) {
-  const r = await fetch(path);
+  const r = await fetch(BASE + path.replace(/^\//, ""));
   if (r.status === 401) {
-    location.href = "/login?next=" + encodeURIComponent(location.pathname + location.search);
+    location.href = BASE + "login?next=" + encodeURIComponent(location.pathname + location.search);
     throw new Error("not authenticated");
   }
   if (!r.ok) {
@@ -40,7 +46,7 @@ async function refreshHistory() {
   try { history = await api("/api/history"); } catch { /* home just shows empty */ }
 }
 function postHistory(entry) {
-  fetch("/api/history", {method: "POST", headers: {"Content-Type": "application/json"},
+  fetch(BASE + "api/history", {method: "POST", headers: {"Content-Type": "application/json"},
     body: JSON.stringify(entry)}).catch(() => {});
 }
 const fmtPos = s => { s = Math.floor(s || 0); return `${Math.floor(s / 60)}:${String(s % 60).padStart(2, "0")}`; };
@@ -222,12 +228,12 @@ function showPlayOverlay(v) {
     document.getElementById("me").textContent = "hi, " + me.user;
     const lo = document.getElementById("logout");
     lo.style.display = ""; lo.onclick = async () => {
-      await fetch("/logout", {method: "POST"}); location.href = "/login";
+      await fetch(BASE + "logout", {method: "POST"}); location.href = BASE + "login";
     };
     const da = document.getElementById("delacct");
     da.style.display = ""; da.onclick = async () => {
       if (!confirm("Delete your account and watch history?")) return;
-      await fetch("/account/delete", {method: "POST"}); location.href = "/login";
+      await fetch(BASE + "account/delete", {method: "POST"}); location.href = BASE + "login";
     };
   } catch { /* not logged in: api() already redirected to /login */ }
   await refreshHistory();