Implement maintenance mode and presence updates in bot and WebUI

- Added maintenance mode functionality in the bot, allowing for a custom message and DND status when enabled.
- Updated the OwnerPresencePage to include read-only access for VIEWER roles, with save permissions restricted to ADMIN roles.
- Enhanced the PresenceForm component to support immediate application of changes and added a confirmation for maintenance mode activation.
- Introduced localization updates for presence-related messages in both English and German, improving user guidance.
- Implemented a presence refresh queue to handle updates efficiently after configuration changes.

apps/webui/src/lib/welcome-preview-vars.ts

@@ -0,0 +1,40 @@
+import type { DashboardGuild, SessionUser } from '@nexumi/shared';
+
+function discordDefaultAvatarUrl(userId: string): string {
+  try {
+    const index = Number((BigInt(userId) >> 22n) % 6n);
+    return `https://cdn.discordapp.com/embed/avatars/${index}.png`;
+  } catch {
+    return 'https://cdn.discordapp.com/embed/avatars/0.png';
+  }
+}
+
+function userAvatarUrl(user: SessionUser): string {
+  return user.avatar
+    ? `https://cdn.discordapp.com/avatars/${user.id}/${user.avatar}.png?size=256`
+    : discordDefaultAvatarUrl(user.id);
+}
+
+function guildIconUrl(guild: DashboardGuild): string {
+  return guild.icon
+    ? `https://cdn.discordapp.com/icons/${guild.id}/${guild.icon}.png?size=128`
+    : discordDefaultAvatarUrl(guild.id);
+}
+
+/** Preview placeholders from the logged-in dashboard user and current guild. */
+export function buildWelcomePreviewVars(
+  user: SessionUser,
+  guild: DashboardGuild
+): Record<string, string> {
+  const displayName = user.globalName?.trim() || user.username;
+  return {
+    user: `@${displayName}`,
+    'user.name': displayName,
+    'user.tag': user.username,
+    'user.id': user.id,
+    'user.avatar': userAvatarUrl(user),
+    server: guild.name,
+    'server.icon': guildIconUrl(guild),
+    memberCount: '—'
+  };
+}