[helm] Support multi-disk data.dirs in the tablet StatefulSet

### Motivation

Fluss 1.0 supports local multi-disk (JBOD) storage via `data.dirs`, which supersedes `data.dir`: tablet servers place new tablets on the least-loaded disk, balancing log and KV storage independently.

The Helm chart cannot express this today:

- `helm/templates/sts-tablet.yaml` renders exactly one `volumeClaimTemplates` entry (named `data`, sized by `tablet.storage.size`).
- The mount path `/tmp/fluss/data` is hardcoded in the template and mirrored by `data.dir: /tmp/fluss/data` in `values.yaml`.

So a chart-deployed tablet server is limited to a single PVC per pod and cannot benefit from JBOD placement.

### Proposal

1. Drive `volumeClaimTemplates` from a values list, so each entry gets its own name, size, and storage class, for example:

```yaml
tablet:
storage:
volumes:
- name: data-0
size: 100Gi
storageClass: fast
- name: data-1
size: 100Gi
storageClass: fast
```

2. Derive the container `volumeMounts` paths from the same list instead of the hardcoded `/tmp/fluss/data`.
3. Emit `data.dirs` (comma-separated mount paths) in the rendered server configuration instead of `data.dir` when more than one volume is configured.

Backward compatibility: a single-volume configuration keeps rendering today's single PVC and `data.dir`, so existing installations upgrade without a StatefulSet storage change.

### Affected files

- `helm/templates/sts-tablet.yaml` (`volumeClaimTemplates` loop, `volumeMounts`)
- `helm/values.yaml` (volume list, `data.dirs`)
- `helm/templates/configmap.yaml` / config rendering (emit `data.dirs`)
- `helm/tests/` (unit tests for single- and multi-disk rendering)

Start with helm/templates/sts-tablet.yaml and helm/values.yaml to trace the current single-volume and data.dir rendering, then inspect helm/templates/configmap.yaml and the existing helm/tests/. Done means single-volume values preserve the current PVC and data.dir output, while multi-volume values render one claim and mount per entry, use data.dirs, and have unit coverage.

Written by the indexing model from the issue text.