Enhance dashboard overview with activity summary and sidebar commands

- Integrated activity summary into the Guild Overview page, displaying message count, voice minutes, and active user count for the last 7 days.
- Updated sidebar to include a new "Commands" link for easier navigation.
- Added corresponding localization keys for activity metrics in both English and German JSON files.

apps/webui/src/lib/activity.ts

@@ -0,0 +1,31 @@
+import type { ActivityStatSummary } from '@nexumi/shared';
+import { prisma } from './prisma';
+
+const SUMMARY_WINDOW_DAYS = 7;
+
+/**
+ * Aggregates `ActivityStat` rows (written by the bot's leveling/stats
+ * modules) for the last {@link SUMMARY_WINDOW_DAYS} days into totals shown
+ * on the guild overview page.
+ */
+export async function getActivitySummary(guildId: string): Promise<ActivityStatSummary> {
+  const since = new Date(Date.now() - SUMMARY_WINDOW_DAYS * 24 * 60 * 60 * 1000);
+
+  const [aggregate, activeUsers] = await Promise.all([
+    prisma.activityStat.aggregate({
+      where: { guildId, day: { gte: since } },
+      _sum: { messageCount: true, voiceMinutes: true }
+    }),
+    prisma.activityStat.findMany({
+      where: { guildId, day: { gte: since } },
+      distinct: ['userId'],
+      select: { userId: true }
+    })
+  ]);
+
+  return {
+    messageCount: aggregate._sum.messageCount ?? 0,
+    voiceMinutes: aggregate._sum.voiceMinutes ?? 0,
+    activeUserCount: activeUsers.length
+  };
+}