feat(dashboard): implement Phase 7 router registration and config setup

Add DashboardService and DashboardHandler to application configuration:

Router Config Updates (internal/router/router.go):
- Add services import for DashboardService type
- Add DashboardService field to Config struct
- DashboardService: Used by SSR routes in frontend.go for data fetching
- DashboardHandler: Used by API routes in dashboard.go for JSON endpoints

Server Initialization (cmd/server/main.go):
- Create dashboardService instance using services.NewDashboardService(queries)
- Keep dashboardHandler creation (already exists from Phase 4)
- Add DashboardService to routerConfig
- Both services now available for dependency injection

Test Helpers (cmd/server/tests/test_helpers.go):
- Create dashboardService instance for testing
- Create dashboardHandler instance for testing
- Add both DashboardService and DashboardHandler to routerConfig
- Ensures test environment matches production setup

Architecture Rationale:
- DashboardService: Service layer with business logic (reusable by SSR, mobile)
- DashboardHandler: HTTP handler layer (JSON API endpoints)
- Separation allows SSR templates to call service directly
- API routes use handler for proper HTTP response handling
- Mobile apps can use API endpoints via DashboardHandler

All three files updated consistently for complete integration.

internal/router/router.go

@@ -6,6 +6,7 @@ import (
 	"bookhoard/internal/handlers"
 	"bookhoard/internal/middleware"
 	ratelimit "bookhoard/internal/middleware"
+	"bookhoard/internal/services"
 	"bookhoard/internal/sync"
 	"log"
 	"net/http"
@@ -48,6 +49,7 @@ type Config struct {
 	QueueHandler          *handlers.QueueHandler
 	CollectionHandler     *handlers.CollectionHandler
 	DashboardHandler      *handlers.DashboardHandler
+	DashboardService      *services.DashboardService
 	OPDSHandler           *handlers.OPDSHandler
 	SystemSettingsHandler *handlers.SystemSettingsHandler
 	ConnManager           *sync.ConnectionManager