feat: URL state for shareable filter views

- Dashboard filters (year, district, museum, quarter) sync to URL
- Comparison filters (preset, dates, district, museum) sync to URL
- Users can now share links with specific filter configurations
- Reset filters also clears URL parameters
- Add useUrlState hook (unused but available for future use)

src/components/Dashboard.js

@@ -1,4 +1,5 @@
 import React, { useState, useMemo } from 'react';
+import { useSearchParams } from 'react-router-dom';
 import { Line, Doughnut, Bar } from 'react-chartjs-2';
 import { Carousel, EmptyState, FilterControls, StatCard } from './shared';
 import { chartColors, createBaseOptions } from '../config/chartConfig';
@@ -24,8 +25,35 @@ const defaultFilters = {
   quarter: 'all'
 };
 
+const filterKeys = ['year', 'district', 'museum', 'quarter'];
+
 function Dashboard({ data, showDataLabels }) {
-  const [filters, setFilters] = useState(defaultFilters);
+  const [searchParams, setSearchParams] = useSearchParams();
+  
+  // Initialize filters from URL or defaults
+  const [filters, setFiltersState] = useState(() => {
+    const initial = { ...defaultFilters };
+    filterKeys.forEach(key => {
+      const value = searchParams.get(key);
+      if (value) initial[key] = value;
+    });
+    return initial;
+  });
+
+  // Update both state and URL
+  const setFilters = (newFilters) => {
+    const updated = typeof newFilters === 'function' ? newFilters(filters) : newFilters;
+    setFiltersState(updated);
+    
+    const params = new URLSearchParams();
+    filterKeys.forEach(key => {
+      if (updated[key] && updated[key] !== 'all') {
+        params.set(key, updated[key]);
+      }
+    });
+    setSearchParams(params, { replace: true });
+  };
+
   const [activeStatCard, setActiveStatCard] = useState(0);
   const [activeChart, setActiveChart] = useState(0);
   const [trendGranularity, setTrendGranularity] = useState('week');