Make the NIP-55 signer callback robust against Amber URL rewriting

Amber drops the query string when it rebuilds the callback URL and
appends the signed event directly to the path. The mobile login page now
hands out path-based callback URLs (/auth/mobile/signed/{k1}/) so the
event arrives as the remainder of the path.

The new callback runs in the web middleware group: the signer opens it
in the system browser, which shares cookies with the in-app browser
session, so the flow completes immediately — a bridge page issues the
token and fires the einundzwanzig:// deep link. The LoginKey row is
still written as a fallback for the polling login page.

tests/Feature/Auth/MobileAuthTest.php

@@ -86,6 +86,41 @@ it('rejects a nostr signer callback whose event is bound to a different challeng
     expect(LoginKey::query()->where('k1', $k1)->exists())->toBeFalse();
 });
 
+it('completes the login via the path-based signer callback and shows the bridge page', function () {
+    Queue::fake();
+
+    $k1 = bin2hex(random_bytes(32));
+    [$signedEvent, $npub] = makeSignedMobileNostrEvent($k1);
+
+    // Amber appends the URL-encoded signed event after the trailing slash
+    // and drops any query string from the callback URL.
+    $response = $this
+        ->withSession(['mobile_auth' => ['redirect_uri' => 'einundzwanzig://auth', 'device_name' => 'Pixel 10']])
+        ->get('/auth/mobile/signed/'.$k1.'/'.rawurlencode(json_encode($signedEvent)));
+
+    $response->assertOk()->assertSee('einundzwanzig://auth?token=', false);
+
+    $user = User::query()->where('nostr', $npub)->first();
+    expect($user)->not->toBeNull();
+
+    $token = $user->tokens()->first();
+    expect($token)->not->toBeNull()
+        ->and($token->name)->toBe('Pixel 10');
+
+    expect(LoginKey::query()->where('k1', $k1)->exists())->toBeTrue();
+    Queue::assertPushed(FetchNostrProfileJob::class);
+});
+
+it('rejects a path-based signer callback with a tampered challenge', function () {
+    $k1 = bin2hex(random_bytes(32));
+    [$signedEvent] = makeSignedMobileNostrEvent(bin2hex(random_bytes(32)));
+
+    $this->get('/auth/mobile/signed/'.$k1.'/'.rawurlencode(json_encode($signedEvent)))
+        ->assertRedirect(route('auth.mobile'));
+
+    expect(LoginKey::query()->where('k1', $k1)->exists())->toBeFalse();
+});
+
 it('issues a token and redirects into the app when completing a verified login', function () {
     $user = User::factory()->create();
     $k1 = bin2hex(random_bytes(32));