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/components/modules/welcome-form.tsx

@@ -238,7 +238,7 @@ export function WelcomeForm({ guildId, initialValue }: WelcomeFormProps) {
                 <Textarea
                   value={value.leaveContent ?? ''}
                   onChange={(event) => setValue((prev) => ({ ...prev, leaveContent: event.target.value || null }))}
-                  placeholder={t('modulePages.welcome.contentPlaceholder')}
+                  placeholder={t('modulePages.welcome.leaveContentPlaceholder')}
                 />
                 <PlaceholderHelp preset="welcome" />
               </div>
@@ -251,6 +251,8 @@ export function WelcomeForm({ guildId, initialValue }: WelcomeFormProps) {
                   onChange={(leaveEmbed) => setValue((prev) => ({ ...prev, leaveEmbed }))}
                   previewVars={WELCOME_PREVIEW_VARS}
                   placeholderPreset="welcome"
+                  titlePlaceholder={t('modulePages.welcome.leaveEmbedTitlePlaceholder')}
+                  descriptionPlaceholder={t('modulePages.welcome.leaveEmbedDescriptionPlaceholder')}
                 />
               </div>
               </FieldAnchor>