Add registration_activity chart and enhance admin statistics

- Introduced `RegistrationActivityChart` to display user registration trends over 14 days.
- Enhanced `AdminStatsResponse` with `registration_activity`, providing improved insights for admin users.
- Updated demo data to include realistic registration activity and organization details.
- Refactored admin page to use updated statistics data model and improved query handling.
- Fixed inconsistent timezone handling in statistical analytics and demo user timestamps.

frontend/src/components/charts/RegistrationActivityChart.tsx

@@ -0,0 +1,113 @@
+/**
+ * RegistrationActivityChart Component
+ * Displays user registration activity over time using an area chart
+ */
+
+'use client';
+
+import {
+  AreaChart,
+  Area,
+  XAxis,
+  YAxis,
+  CartesianGrid,
+  Tooltip,
+  ResponsiveContainer,
+  Legend,
+} from 'recharts';
+import { ChartCard } from './ChartCard';
+import { CHART_PALETTES } from '@/lib/chart-colors';
+
+export interface RegistrationActivityData {
+  date: string;
+  registrations: number;
+}
+
+interface RegistrationActivityChartProps {
+  data?: RegistrationActivityData[];
+  loading?: boolean;
+  error?: string | null;
+}
+
+// Custom tooltip with proper theme colors  
+const CustomTooltip = ({ active, payload }: any) => {
+  if (active && payload && payload.length) {
+    return (
+      <div
+        style={{
+          backgroundColor: 'hsl(var(--popover))',
+          border: '1px solid hsl(var(--border))',
+          borderRadius: '6px',
+          padding: '8px 12px',
+        }}
+      >
+        <p style={{ color: 'hsl(var(--popover-foreground))', margin: 0, fontSize: '13px', fontWeight: 600 }}>
+          {payload[0].payload.date}
+        </p>
+        <p style={{ color: 'hsl(var(--popover-foreground))', margin: '4px 0 0 0', fontSize: '12px' }}>
+          New Registrations: {payload[0].value}
+        </p>
+      </div>
+    );
+  }
+  return null;
+};
+
+export function RegistrationActivityChart({
+  data,
+  loading,
+  error,
+}: RegistrationActivityChartProps) {
+  // Show empty chart if no data available
+  const chartData = data || [];
+  const hasData = chartData.length > 0 && chartData.some((d) => d.registrations > 0);
+
+  return (
+    <ChartCard
+      title="User Registration Activity"
+      description="New user registrations over the last 14 days"
+      loading={loading}
+      error={error}
+    >
+      {!hasData && !loading && !error ? (
+        <div className="flex items-center justify-center h-[300px] text-muted-foreground">
+          <p>No registration data available</p>
+        </div>
+      ) : (
+        <ResponsiveContainer width="100%" height={300}>
+          <AreaChart data={chartData} margin={{ top: 5, right: 30, left: 20, bottom: 5 }}>
+            <defs>
+              <linearGradient id="colorRegistrations" x1="0" y1="0" x2="0" y2="1">
+                <stop offset="5%" stopColor={CHART_PALETTES.area[0]} stopOpacity={0.8} />
+                <stop offset="95%" stopColor={CHART_PALETTES.area[0]} stopOpacity={0.1} />
+              </linearGradient>
+            </defs>
+            <CartesianGrid strokeDasharray="3 3" style={{ stroke: 'var(--muted)', opacity: 0.2 }} />
+            <XAxis
+              dataKey="date"
+              style={{ fill: 'var(--muted-foreground)', fontSize: '12px' }}
+            />
+            <YAxis
+              style={{ fill: 'var(--muted-foreground)', fontSize: '12px' }}
+            />
+            <Tooltip content={<CustomTooltip />} />
+            <Legend
+              wrapperStyle={{
+                paddingTop: '20px',
+              }}
+            />
+            <Area
+              type="monotone"
+              dataKey="registrations"
+              name="New Registrations"
+              stroke={CHART_PALETTES.area[0]}
+              strokeWidth={2}
+              fillOpacity={1}
+              fill="url(#colorRegistrations)"
+            />
+          </AreaChart>
+        </ResponsiveContainer>
+      )}
+    </ChartCard>
+  );
+}