Warn the user when using an older, unsupported browser

### Description

We just encountered an issue where a user had a very old version of a browser, not supporting widely available web functionality, causing the app to crash upon loading.

With this feature, we'd like to show a warning in the app whenever the user is on an unsupported browser.

Requirements:
- [ ] Design this message (look, feel, placement, content)
- [ ] Show a non-intrusive warning when using an unsupported browser (possibly only on error pages)
- [ ] Show the 'unknown error' page if the app crashes before displaying anything (this could be extracted out to a separate issue if needs be)
- [ ] Always show the warning about using an unsupported browser on the 'unknown error' page (unexpected errors can occur when using these browsers)

### Additional Information

_No response_

No files or tests are named. Start by locating the app startup path and the unknown-error page, then determine how browser support can be detected before normal rendering; done means an agreed warning design appears for unsupported browsers and the unknown-error page is shown with that warning when loading fails.

Written by the indexing model from the issue text.