fix(viewer): satisfy server lint, psalm, autoloader and openapi checks

Align the vendored Viewer app with the server's stricter checks:
- eslint: camelCase bound attributes, import order, attach `cause` to the
  re-thrown rename error, typed `@throws`
- stylelint: explicit .scss import extension, scope the shared Plyr partial
- php-cs/rector: format the Files event test stub
- register the app in psalm.xml and regenerate its composer autoloader
- mark the app as having no OpenAPI spec (.noopenapi)

Assisted-by: ClaudeCode:claude-opus-4-8
Signed-off-by: skjnldsv <skjnldsv@protonmail.com>

Author: skjnldsv

apps/viewer/.noopenapi

@@ -3,21 +3,18 @@
 // autoload.php @generated by Composer
 
 if (PHP_VERSION_ID < 50600) {
-	if (!headers_sent()) {
-		header('HTTP/1.1 500 Internal Server Error');
-	}
-	$err = 'Composer 2.3.0 dropped support for autoloading on PHP <5.6 and you are running ' . PHP_VERSION . ', please upgrade PHP or use Composer 2.2 LTS via "composer self-update --2.2". Aborting.' . PHP_EOL;
-	if (!ini_get('display_errors')) {
-		if (PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg') {
-			fwrite(STDERR, $err);
-		} elseif (!headers_sent()) {
-			echo $err;
-		}
-	}
-	trigger_error(
-		$err,
-		E_USER_ERROR
-	);
+    if (!headers_sent()) {
+        header('HTTP/1.1 500 Internal Server Error');
+    }
+    $err = 'Composer 2.3.0 dropped support for autoloading on PHP <5.6 and you are running '.PHP_VERSION.', please upgrade PHP or use Composer 2.2 LTS via "composer self-update --2.2". Aborting.'.PHP_EOL;
+    if (!ini_get('display_errors')) {
+        if (PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg') {
+            fwrite(STDERR, $err);
+        } elseif (!headers_sent()) {
+            echo $err;
+        }
+    }
+    throw new RuntimeException($err);
 }
 
 require_once __DIR__ . '/composer/autoload_real.php';