bug: Grafana Store used panel uses store_used instead of capacity-available

## Bug Report

### What did you do?

Checked the PD Grafana dashboard template in this repository and looked at the `Store used` panel query in `metrics/grafana/pd.json`.

Current query:
`pd_scheduler_store_status{..., type="store_used"}`

### What did you expect to see?

`Store used` should be calculated as:
`store_capacity - store_available`

Expected query shape:
`pd_scheduler_store_status{..., type="store_capacity"} - pd_scheduler_store_status{..., type="store_available"}`

This aligns storage-used semantics with actual disk usage definition.

### What did you see instead?

The dashboard uses `store_used` directly, which can diverge from the expected `capacity - available` definition.

### What version of PD are you using (`pd-server -V`)?

master branch (dashboard template in repository: `metrics/grafana/pd.json`)