Cannot manage user dictionary

I confirm (by marking "x" in the [ ] below: [x]):

- [x] This is not a troubleshooting question. [Troubleshooting questions go here: https://docs.mattermost.com/install/troubleshooting.html](https://docs.mattermost.com/install/troubleshooting.html).
- [x] This doesn't reproduce on web browsers (such as in Chrome). If it does, [issue reports go to the Mattermost Server repository](https://github.com/mattermost/platform/issues).
- [x] I have read [contributing guidelines](https://github.com/mattermost/desktop/blob/master/CONTRIBUTING.md).

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**Summary**

Cannot manage spelling dictionary

**Environment**
- Operating System: Win 10
- Mattermost Desktop App version: 5.1.1
- Mattermost Server version: n/a

**Steps to reproduce**

1. Misspell a word
2. Right-click and select "learn spelling"
3. Word now shows as not misspelled with no way to undo addition to user dictionary

**Expected behavior**

1. Misspell a word
2. Right-click and select "learn spelling"
3. Word now shows as not misspelled but user has way to manage user dictionary

**Observed behavior**

Word now shows as not misspelled with no way to undo addition to user dictionary

**Possible fixes**

Add way to manage user dictionary

Start with the reproduction steps in the issue and trace the desktop app's spelling context-menu handling and user-dictionary entry point. Determine how learned words are stored and where a user could manage or remove them. Done means the learned word remains supported while users have a discoverable way to view or remove entries, with the behavior tested.

Written by the indexing model from the issue text.