feat: Implement Pan-EU grouping in Grid tab

- Added applyPanEUGrouping() helper function in dataProcessor.ts
- Groups Amazon DE/IT/FR/ES as 'Pan-EU' when no customer filter applied
- Shows individual countries when specific filter is selected
- Updated DataGrid to accept hasCustomerFilter prop
- Updated App.tsx to pass filter state to DataGrid
- Keeps Amazon UK and Amazon SC separate
- Verified with browser testing: grouping works correctly

App.tsx

@@ -363,7 +363,7 @@ const App: React.FC = () => {
                   contextData={contextAggregatedData}
                 />
               )}
-              {view === 'table' && <DataGrid data={filteredData} />}
+              {view === 'table' && <DataGrid data={filteredData} hasCustomerFilter={filters.customer.length > 0} />}
               {view === 'movers' && <TopMovers data={filteredData} />}
             </div>
           </>