refactor: read timezone from shell-level site_timezone

Drops unifi.timezone from the settings form (now lives in
Admin → Settings on the shell). Schedulers (PPSK sync, password
rotation) now read \App\Support\Timezone::current() — same fallback
chain as the rest of the platform.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Console/SyncPpskSchedules.php

@@ -18,7 +18,7 @@ class SyncPpskSchedules extends Command
             return self::SUCCESS;
         }
 
-        $tz   = Setting::get('unifi.timezone', 'UTC');
+        $tz   = \App\Support\Timezone::current();
         $now  = now($tz);
         $day  = $now->dayOfWeek;                                 // 0=Sun … 6=Sat
         $slot = $now->hour * 2 + ($now->minute >= 30 ? 1 : 0);  // 0–47