[Code scan] Restore the saved language from storage instead of overwriting it

This issue was found during a Codex global code scan of the repository.

Baseline commit: e3c5b38a99eb7ab778b5d8c68a5ed4ddf6cf91b3

Problem

The mounted hook calls the storage setter when it appears to intend to read the saved language.

Code references:
https://github.com/deepmodeling/dpgui/blob/e3c5b38a99eb7ab778b5d8c68a5ed4ddf6cf91b3/src/App.vue#L101-L103
https://github.com/deepmodeling/dpgui/blob/e3c5b38a99eb7ab778b5d8c68a5ed4ddf6cf91b3/src/App.vue#L208-L211

Relevant snippet:
```js
this.$i18n.locale =
this.$storage.setStorageSync("lang") || this.$i18n.locale;
```

Impact

A previously selected language is not restored on reload. Depending on the storage helper behavior, the call may also overwrite the saved `lang` value with an empty value.

Suggested fix

Use `getStorageSync("lang")` in `mounted()` and keep `setStorageSync("lang", value)` only in the language update handler.