feat: replace year/quarter filters with free date range pickers

Dashboard: PeriodPicker replaces year + quarter dropdowns. Defaults to
current month. YoY stat card now compares same range vs previous year.

Comparison: two independent PeriodPicker blocks (Period A and Period B).
Changing Period A auto-updates Period B to same period previous year,
but Period B remains freely editable.

Both pages use filterDataByDateRange; Filters type drops year/quarter.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/services/dataService.ts

@@ -273,11 +273,9 @@ export async function refreshData(): Promise<FetchResult> {
 
 export function filterData(data: MuseumRecord[], filters: Filters): MuseumRecord[] {
   return data.filter(row => {
-    if (filters.year && filters.year !== 'all' && row.year !== filters.year) return false;
     if (filters.district && filters.district !== 'all' && row.district !== filters.district) return false;
     if (filters.channel.length > 0 && !filters.channel.includes(row.channel)) return false;
     if (filters.museum.length > 0 && !filters.museum.includes(row.museum_name)) return false;
-    if (filters.quarter && filters.quarter !== 'all' && row.quarter !== filters.quarter) return false;
     return true;
   });
 }