Global gantt chart (across all DAGs)

### Description

Hi, now we have a Gantt view inside each DAG which helps to understand the overlapping/idle times/etc on each one. The request is for a global Gantt across all DAGs

### Use case/motivation

This will allow knowing where Airflow and all the services that depend on it will have idle times, maybe look at the chart by tag. Optimizations on a lot of fronts come to mind when I think of this feature.

### Related issues

_No response_

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

Start by locating the existing Gantt view for an individual DAG and tracing how its data is assembled and displayed. Determine how a global view could combine all DAGs and support filtering by tag; done means the chart represents cross-DAG scheduling activity and exposes the requested idle or overlap information.

Written by the indexing model from the issue text.