feat: instant preview of edited email template

src/Config/EmailTextDefinition.php

@@ -28,4 +28,18 @@ readonly class EmailTextDefinition
     {
         return array_keys($this->placeholders);
     }
+
+    /**
+     * Placeholder values for the admin preview, where the placeholders stand in for
+     * themselves: no disposition is at hand there, and a visible [destination] tells an
+     * admin where a real value lands better than an invented one would.
+     *
+     * @return array<string, string>
+     */
+    public function getSamplePlaceholders(): array
+    {
+        $names = $this->getPlaceholderNames();
+
+        return array_combine($names, array_map(static fn (string $name): string => '['.$name.']', $names));
+    }
 }