Refactor OnboardingView to functional component with hooks (#8752)

Refactor the `<OnboardingView>` to a functional React component with
hooks.

### URL of deployed dev instance (used for testing):
- https://refactoronboarding.webknossos.xyz/

### Steps to test:
- Verify that the onboarding flow functions as expected.
- Ensure all modals display correctly and user interactions work as
intended.

### Issues:
- Contributes to #8747

------
(Please delete unneeded items, merge only when none are left open)
- [ ] Added changelog entry (create a `$PR_NUMBER.md` file in
`unreleased_changes` or use `./tools/create-changelog-entry.py`)
- [ ] Added migration guide entry if applicable (edit the same file as
for the changelog)
- [ ] Updated [documentation](../blob/master/docs) if applicable
- [ ] Adapted [wk-libs python
client](https://github.com/scalableminds/webknossos-libs/tree/master/webknossos/webknossos/client)
if relevant API parts change
- [ ] Removed dev-only changes like prints and application.conf edits
- [ ] Considered [common edge
cases](../blob/master/.github/common_edge_cases.md)
- [ ] Needs datastore update after deployment

Author: hotzenklotz