fix(settings): wp_date() respects WP timezone; clarify counter is not atomic

- Replace date('Y-m-d') with wp_date('Y-m-d') in increment_drop_counter and
  default_drop_counters to respect WordPress timezone settings.
- Update corresponding test assertions to use wp_date() for consistency.
- Clarify in docblock that increment_drop_counter is not atomic under concurrent
  submissions; read-modify-write pattern via update_option is acceptable for
  low-volume dashboard counters.

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

includes/class-settings.php

@@ -125,15 +125,18 @@ final class Settings
     }
 
     /**
-     * Atomically increments the drop counter. `$mode` must be 'real' or 'dry_run';
+     * Increments the drop counter. `$mode` must be 'real' or 'dry_run';
      * any other value is a no-op (defensive — guards against typos in callers).
      * Performs rolling today→yesterday on date change.
+     *
+     * Note: not atomic under concurrent CF7 submissions — `update_option` is a
+     * read-modify-write. Acceptable for low-volume dashboard counters.
      */
     public static function increment_drop_counter(string $mode): void
     {
         if (!in_array($mode, ['real', 'dry_run'], true)) return;
         $c = self::get_drop_counters();
-        $today = date('Y-m-d');
+        $today = wp_date('Y-m-d');
         if ($c['today_date'] !== $today) {
             $c['real_yesterday'] = $c['real_today'];
             $c['dry_yesterday']  = $c['dry_today'];
@@ -157,7 +160,7 @@ final class Settings
 
     private static function default_drop_counters(): array
     {
-        $today = date('Y-m-d');
+        $today = wp_date('Y-m-d');
         return [
             'total'          => 0,
             'real_today'     => 0,