Add single-user handling (#9)

- Disallow log on for single-user mode user
- Improve CLI header display

src/cli-start.php

@@ -22,5 +22,9 @@ function printfn(string $format, mixed ...$values): void {
  * @param string $title The title to display on the command line
  */
 function cli_title(string $title): void {
-    printfn("$title | Feed Reader Central v%s" . PHP_EOL, FRC_VERSION);
+    $appTitle = 'Feed Reader Central ~ v' . FRC_VERSION;
+    $dashes   = '  +' . str_repeat('-', strlen($title) + 2) . '+' . str_repeat('-', strlen($appTitle) + 2) . '+';
+    printfn($dashes);
+    printfn('  | %s | %s |', $title, $appTitle);
+    printfn($dashes . PHP_EOL);
 }