[Code scan] list-valued spring_k crashes einstein.free_energy

Source: Codex global repository scan of deepmodeling/dpti at commit b719828e7eeb571bd26411430197cd74ea78e38c.
Project: https://github.com/orgs/deepmodeling/projects/27

Problem
`einstein.free_energy()` supports scalar `spring_k` by constructing `m_spring_k`, but if `spring_k` is already a list, `m_spring_k` is never initialized and is immediately referenced. This makes list-valued spring constants unusable in the Vega Einstein free-energy path.

Code references
https://github.com/deepmodeling/dpti/blob/b719828e7eeb571bd26411430197cd74ea78e38c/dpti/einstein.py#L70
https://github.com/deepmodeling/dpti/blob/b719828e7eeb571bd26411430197cd74ea78e38c/dpti/einstein.py#L71
https://github.com/deepmodeling/dpti/blob/b719828e7eeb571bd26411430197cd74ea78e38c/dpti/einstein.py#L76

Reproduction
Create a minimal job with `in.json` containing a list-valued `spring_k`, such as `"spring_k": [1.0]`, then run:

```python
from dpti.einstein import free_energy
free_energy('JOB')
```

Expected result
List-valued spring constants should be accepted directly or validated with a clear error. They should not raise `UnboundLocalError` from an uninitialized local variable.