obsidian plugin - local spelling dictionary file

**What problem does this solve?**

The "personal dictionary" is a section in the json config file. So every word that's specific to the vault (technical terms, names, etc) needs to be manually added to harper as well as having a shared file for other tools.

**Proposed Solution**
Add selecting a local file to load as a word list.

**Component**
- [ ] Core engine
- [x] Plugin/Extension
- [ ] Other: _____

Start by locating the Obsidian plugin/extension component and the existing personal dictionary handling in the JSON configuration. Trace how dictionary words are loaded, then implement local-file selection so vault-specific words can be loaded; done means a selected local word list is available to the plugin alongside the existing dictionary.

Written by the indexing model from the issue text.