fix(web): address auth review findings (open redirect, dead code, HMAC per-req)

Pre-merge review of the P0 branch:
- Open redirect: login redirected to an unvalidated ?next param, so a crafted
  /login?next=https://evil (or //evil) bounced off-site after login. Now only
  same-origin relative paths are honored (else fall back to "/").
- Remove the unused useRouter import/binding in the login form.
- Memoize expectedToken() keyed on (password, secret) so middleware stops
  recomputing an HMAC on every gated request.

Deferred (low-value/latent, noted): Soulseek cover.jpg isn't retrieved into
staging (UI still shows Cover Art Archive art); _retrieve's basename-keyed map
could undercount same-named files but source_refs come from a single slskd dir;
SignOut visibility rides a server-env read (cosmetic).

web 138 tests green, tsc + build clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/lib/auth.ts

@@ -13,11 +13,16 @@ function toB64(bytes: Uint8Array): string {
   return btoa(s);
 }
 
+// Memoize the derived token so the middleware doesn't recompute an HMAC on every request
+// (password/secret don't change at runtime; keyed on both so a change still recomputes).
+let _cache: { password: string; secret: string; token: string } | null = null;
+
 /** The token a valid session cookie must carry, or "" when auth is disabled (no password). */
 export async function expectedToken(): Promise<string> {
   const password = process.env.LYRA_PASSWORD;
   if (!password) return "";
   const secret = process.env.LYRA_SECRET_KEY ?? "lyra";
+  if (_cache && _cache.password === password && _cache.secret === secret) return _cache.token;
   const enc = new TextEncoder();
   const key = await crypto.subtle.importKey(
     "raw",
@@ -27,7 +32,9 @@ export async function expectedToken(): Promise<string> {
     ["sign"],
   );
   const sig = await crypto.subtle.sign("HMAC", key, enc.encode(password));
-  return toB64(new Uint8Array(sig));
+  const token = toB64(new Uint8Array(sig));
+  _cache = { password, secret, token };
+  return token;
 }
 
 /** Constant-time string compare (both operands are our own derived tokens). */