Update placeholders in WelcomeForm and DiscordEmbedBuilder for improved user guidance

- Changed placeholder text in WelcomeForm to use more descriptive keys for leave content.
- Enhanced DiscordEmbedBuilder to accept custom title and description placeholders, improving flexibility for embed creation.
- Updated localization files in English and German to include new placeholder texts, ensuring consistency and clarity for users.

apps/webui/src/messages/en.json

@@ -361,7 +361,10 @@
       "leaveEnabledLabel": "Leave message enabled",
       "leaveChannelId": "Leave channel ID",
       "leaveContent": "Message content",
-      "leaveEmbed": "Embed"
+      "leaveContentPlaceholder": "Goodbye {user.tag} — see you around on {server}!",
+      "leaveEmbed": "Embed",
+      "leaveEmbedTitlePlaceholder": "Goodbye",
+      "leaveEmbedDescriptionPlaceholder": "{user.tag} left {server}."
     },
     "verification": {
       "title": "Verification",