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/Comparison.js

@@ -1,4 +1,5 @@
 import React, { useState, useMemo, useCallback, useRef } from 'react';
+import { useSearchParams } from 'react-router-dom';
 import { Line, Bar } from 'react-chartjs-2';
 import { EmptyState, FilterControls } from './shared';
 import { chartColors, createBaseOptions } from '../config/chartConfig';
@@ -38,17 +39,77 @@ const generatePresetDates = (year) => ({
 });
 
 function Comparison({ data, showDataLabels }) {
+  const [searchParams, setSearchParams] = useSearchParams();
+  
   // Get latest year from data for default presets
   const latestYear = useMemo(() => getLatestYear(data), [data]);
+  const presetDates = useMemo(() => generatePresetDates(latestYear), [latestYear]);
+
+  // Initialize state from URL or defaults
+  const [preset, setPresetState] = useState(() => searchParams.get('preset') || 'jan');
+  const [startDate, setStartDateState] = useState(() => {
+    const urlPreset = searchParams.get('preset');
+    if (urlPreset && urlPreset !== 'custom' && presetDates[urlPreset]) {
+      return presetDates[urlPreset].start;
+    }
+    return searchParams.get('from') || `${latestYear}-01-01`;
+  });
+  const [endDate, setEndDateState] = useState(() => {
+    const urlPreset = searchParams.get('preset');
+    if (urlPreset && urlPreset !== 'custom' && presetDates[urlPreset]) {
+      return presetDates[urlPreset].end;
+    }
+    return searchParams.get('to') || `${latestYear}-01-31`;
+  });
+  const [filters, setFiltersState] = useState(() => ({
+    district: searchParams.get('district') || 'all',
+    museum: searchParams.get('museum') || 'all'
+  }));
   
-  const [preset, setPreset] = useState('jan');
-  const [startDate, setStartDate] = useState(`${latestYear}-01-01`);
-  const [endDate, setEndDate] = useState(`${latestYear}-01-31`);
-  const [filters, setFilters] = useState({ district: 'all', museum: 'all' });
   const [chartMetric, setChartMetric] = useState('revenue');
   const [chartGranularity, setChartGranularity] = useState('week');
   const [activeChart, setActiveChart] = useState(0);
   const [activeCard, setActiveCard] = useState(0);
+
+  // Update URL with current state
+  const updateUrl = useCallback((newPreset, newFrom, newTo, newFilters) => {
+    const params = new URLSearchParams();
+    if (newPreset && newPreset !== 'jan') params.set('preset', newPreset);
+    if (newPreset === 'custom') {
+      if (newFrom) params.set('from', newFrom);
+      if (newTo) params.set('to', newTo);
+    }
+    if (newFilters?.district && newFilters.district !== 'all') params.set('district', newFilters.district);
+    if (newFilters?.museum && newFilters.museum !== 'all') params.set('museum', newFilters.museum);
+    setSearchParams(params, { replace: true });
+  }, [setSearchParams]);
+
+  const setPreset = (value) => {
+    setPresetState(value);
+    if (value !== 'custom' && presetDates[value]) {
+      setStartDateState(presetDates[value].start);
+      setEndDateState(presetDates[value].end);
+      updateUrl(value, null, null, filters);
+    }
+  };
+
+  const setStartDate = (value) => {
+    setStartDateState(value);
+    setPresetState('custom');
+    updateUrl('custom', value, endDate, filters);
+  };
+
+  const setEndDate = (value) => {
+    setEndDateState(value);
+    setPresetState('custom');
+    updateUrl('custom', startDate, value, filters);
+  };
+
+  const setFilters = (newFilters) => {
+    const updated = typeof newFilters === 'function' ? newFilters(filters) : newFilters;
+    setFiltersState(updated);
+    updateUrl(preset, startDate, endDate, updated);
+  };
   
   const charts = [
     { id: 'timeseries', label: 'Trend' },
@@ -103,17 +164,6 @@ function Comparison({ data, showDataLabels }) {
   const districtMuseumMap = useMemo(() => getDistrictMuseumMap(data), [data]);
   const availableMuseums = useMemo(() => getMuseumsForDistrict(districtMuseumMap, filters.district), [districtMuseumMap, filters.district]);
 
-  // Generate presets based on latest year
-  const presetDates = useMemo(() => generatePresetDates(latestYear), [latestYear]);
-
-  const handlePresetChange = (newPreset) => {
-    setPreset(newPreset);
-    if (newPreset !== 'custom' && presetDates[newPreset]) {
-      setStartDate(presetDates[newPreset].start);
-      setEndDate(presetDates[newPreset].end);
-    }
-  };
-
   // Year-over-year comparison: same dates, previous year
   const ranges = useMemo(() => ({
     curr: { start: startDate, end: endDate },
@@ -341,7 +391,7 @@ function Comparison({ data, showDataLabels }) {
       <FilterControls title="Select Period" onReset={resetFilters}>
         <FilterControls.Row>
           <FilterControls.Group label="Preset">
-            <select value={preset} onChange={e => handlePresetChange(e.target.value)}>
+            <select value={preset} onChange={e => setPreset(e.target.value)}>
               <option value="custom">Custom</option>
               <option value="jan">January</option>
               <option value="feb">February</option>

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');