feat(router): Pass initial filter data to page

- Pass data via state.extra
- Simplify SourceFilterPage init

Author: fulleni

lib/router/router.dart

@@ -400,7 +400,21 @@ GoRouter createRouter({
                                         context
                                             .read<HtDataRepository<Country>>(),
                                   ),
-                              child: const SourceFilterPage(),
+                              // Pass initialSelectedSources, country ISO codes, and source types from state.extra
+                              child: Builder(
+                                builder: (context) {
+                                  final extraData = state.extra as Map<String, dynamic>? ?? const {};
+                                  final initialSources = extraData[keySelectedSources] as List<Source>? ?? const [];
+                                  final initialCountryIsoCodes = extraData[keySelectedCountryIsoCodes] as Set<String>? ?? const {};
+                                  final initialSourceTypes = extraData[keySelectedSourceTypes] as Set<SourceType>? ?? const {};
+
+                                  return SourceFilterPage(
+                                    initialSelectedSources: initialSources,
+                                    initialSelectedCountryIsoCodes: initialCountryIsoCodes,
+                                    initialSelectedSourceTypes: initialSourceTypes,
+                                  );
+                                },
+                              ),
                             ),
                       ),
                       // Sub-route for country selection REMOVED