feat(dashboard): register dashboard API routes

Add dashboard route registration and wire up handler:

Router Changes:
- Add DashboardHandler to router.Config struct
- Create internal/router/dashboard.go with dashboard route registration
- Register dashboard routes in main RegisterRoutes function

Dashboard Routes (all protected by JWT):
- GET /api/dashboard/sections: Get dashboard sections for user
  * Query params: library_id (required), limit (optional, default 20, max 100)
  * Returns: JSON with sections array
- PUT /api/dashboard/preferences: Update dashboard preferences
  * Body: library_id, hidden_collections, collection_order, items_per_section
  * Returns: Updated preferences
- POST /api/dashboard/restore-system-collection: Restore system collection to defaults
  * Body: collection_name (must be valid system collection)
  * Returns: Success message

Server Integration:
- Create dashboardHandler in cmd/server/main.go
- Add dashboardHandler to routerConfig
- Routes are automatically registered on server startup

internal/router/dashboard.go

@@ -0,0 +1,13 @@
+package router
+
+func registerDashboardRoutes(cfg *Config) {
+	e := cfg.Echo
+
+	jwtMiddleware := createJWTMiddleware(cfg)
+	protected := e.Group("/api", jwtMiddleware)
+
+	dashboard := protected.Group("/dashboard")
+	dashboard.GET("/sections", cfg.DashboardHandler.GetSections)
+	dashboard.PUT("/preferences", cfg.DashboardHandler.UpdatePreferences)
+	dashboard.POST("/restore-system-collection", cfg.DashboardHandler.RestoreSystemCollection)
+}