Add NoOp email service, /docs shortcut and fix streak timezone bug

Replace nil email service pattern with NoOpEmailService (Null Object)
to eliminate nil pointer panics across all handlers.

Add /docs route as a shortcut to Swagger UI.

Fix streak calculation returning 0 when server timezone differs from
UTC — CreatedAt was not converted to UTC before date extraction.

internal/infrastructure/http/router.go

@@ -30,6 +30,9 @@ func NewRouter(appURL string, habitHandlers *HabitHandlers, authHandlers *AuthHa
 		AllowCredentials: true,
 	}))
 
+	r.Get("/docs", httpSwagger.Handler(
+		httpSwagger.URL("/api/v1/docs/doc.json"),
+	))
 	r.Get("/api/v1/docs", func(w http.ResponseWriter, r *http.Request) {
 		http.Redirect(w, r, "/api/v1/docs/index.html", http.StatusMovedPermanently)
 	})