Add a Usage Analytics Dashboard

Build a usage analytics page at /dashboard/analytics. Display three charts side by side: a line chart showing daily active users over the last 30 days, a bar chart showing feature usage broken down by category, and a pie chart showing signup source distribution (e.g., organic, referral, paid). Use realistic placeholder data for all three charts for now — do not leave them empty. Add a date range picker at the top of the page that filters all three charts simultaneously when the range changes. Below the charts, show a summary table of the same data in tabular form. Include a "Download CSV" button in the top-right corner that exports the currently filtered data as a CSV file with headers. Use a charting library already available in the project or install Recharts if none exists.