feat(auth): require signed NIP-42 event for Nostr login

Closes a security flaw where the server trusted any pubkey the client
sent. The frontend now signs a per-session, time-bound challenge
(kind-22242 event) that the backend verifies with swentel/nostr-php
before establishing the session.

- NostrAuth: issueChallenge() + loginWithSignedEvent() with full
  schnorr/id verification, TTL window, and idempotent re-entry for
  concurrent Livewire listeners.
- auth-button: mounts a fresh challenge, exposes it via data-attribute
  + requestNostrChallenge() fallback, renders a full-viewport AAA-style
  loading overlay while the wallet signs.
- NostrSessionGuard: override logout() to drop the cookie-jar dep so
  programmatic logout works in any context.

tests/Feature/RateLimitingTest.php

@@ -53,14 +53,14 @@ test('voting actions are rate limited after 10 attempts', function () {
 });
 
 test('nostr login is rate limited after 10 attempts', function () {
-    $pleb = EinundzwanzigPleb::factory()->create();
-
     for ($i = 0; $i < 10; $i++) {
         RateLimiter::attempt('nostr-login:127.0.0.1', 10, function () {});
     }
 
+    // Rate limit is checked before signature verification, so an empty
+    // payload is enough to trigger the 429.
     Livewire::test('association.project-support.index')
-        ->call('handleNostrLogin', $pleb->pubkey)
+        ->call('handleNostrLogin', [])
         ->assertStatus(429);
 });