[UX] Year filter — sidebar dropdown to recompute charts by year

[wi287g]

Let users narrow charts to a specific year without reloading.

Tasks

• Add <select id="year-filter"> to sidebar or toolbar, populated from available years in loaded data
• On change, call updateCharts(fips, ...) with a filtered slice of time-series data
• Show "No data for selected year" message when series is empty (toggle #no-chart-data)
• Optional: update map choropleth shade intensity based on detainer rate for that year