Remove stats tab switcher, drop duration-per-device, show devices inline

Sessions and Devices no longer live behind a Tabs switcher on the stats
page - both sections render directly, one after the other. Also drops
the "Duration per device" list from the sessions summary cards since
DeviceUsageTable already shows per-device active time in more detail.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WzyfRyA7h5rs5SCAahLAWd

lib/db/queries/stats.ts

@@ -15,18 +15,7 @@ export async function getSessionsSummary() {
     .from(sessions)
     .where(eq(sessions.status, "completed"));
 
-  const durationPerDevice = await db
-    .select({
-      deviceId: devices.id,
-      displayName: devices.displayName,
-      bleName: devices.bleName,
-      totalActiveMs: sql<number>`coalesce(sum(coalesce(${sessionDevices.disconnectedAt}, ${sessionDevices.connectedAt}) - ${sessionDevices.connectedAt}), 0)`,
-    })
-    .from(sessionDevices)
-    .innerJoin(devices, eq(sessionDevices.deviceId, devices.id))
-    .groupBy(devices.id);
-
-  return { ...totals, durationPerDevice };
+  return totals;
 }
 
 export async function getSessionTimeline(sessionId: number, bucketMs = 1000) {