test(series): add unit and integration tests for series feature

Unit tests:
- series_service_test.go: test continueSeriesRowToMediaItems conversion
  with valid fields, null fields, and comprehensive field mapping
- series_test.go: test handler initialization and textToString helper

Integration tests (series_integration_test.go):
- GET /api/series: requires library_id, rejects invalid UUID, returns
  empty array for empty library, pagination params, limit clamped to
  100, response structure validation, special characters in names
- GET /api/series/books: requires library_id and name, handles
  nonexistent series, unauthorized access
- Restore Continue Series system collection
- Dashboard sections include all 5 collections (including continue-series)

Update test helpers:
- Add SeriesHandler to setupTestServer router config
- Add 5th Continue Series collection to createDefaultCollectionsForUser
- Update dashboard integration test for 5 collections

cmd/server/tests/test_helpers_test.go

@@ -286,6 +286,7 @@ func createDefaultCollectionsForUser(t *testing.T, db *database.Queries, userID
 		{"recently-added", "Newly added items to this library", "🆕", "#9ece6a", "recently-added", 2},
 		{"recently-read", "Books you've finished (progress >= 1)", "✅", "#e0af68", "recently-read", 3},
 		{"not-started", "Books you haven't read yet (progress = 0 or no record)", "📕", "#f7768e", "not-started", 4},
+		{"continue-series", "Next book in series you're reading", "📚", "#bb9af7", "continue-series", 5},
 	}
 
 	for _, col := range defaultCollections {
@@ -478,6 +479,7 @@ func setupTestServer(t *testing.T) *TestServerSetup {
 	filtersHandler := handlers.NewFiltersHandler(queries)
 	dashboardService := services.NewDashboardService(queries)
 	dashboardHandler := handlers.NewDashboardHandler(queries)
+	seriesHandler := handlers.NewSeriesHandler(queries)
 	mediaHandler := handlers.NewMediaHandler(queries, libraryService, worker)
 	mediaHandler.SetProgressService(progressService)
 	matchingHandler := handlers.NewMatchingHandler(queries, connManager)
@@ -529,6 +531,7 @@ func setupTestServer(t *testing.T) *TestServerSetup {
 		FiltersHandler:        filtersHandler,
 		DashboardHandler:      dashboardHandler,
 		DashboardService:      dashboardService,
+		SeriesHandler:         seriesHandler,
 		OPDSHandler:           opdsHandler,
 		JobsHandler:           jobsHandler,
 		ConnManager:           connManager,