This PR updates React Router (RR) from v5 to v6 following their [upgrade
guide](https://reactrouter.com/6.30.1/upgrading/v5). There is already
react-router v7 but let's go step by step. v7 also seems to have the
same API as v6 so we might update soon in a follow up PR (and double
check compatibility with React v18/19).

Noteworthy changes and high level concepts:
- RR v6 offer various ways of setting up a router
([Docs](https://reactrouter.com/6.30.1/routers/picking-a-router)):
  - Declarative: Using JSX `<Route`> elements like we did so far
- Data API: Create routes as TS code. This is the recommend method and
what I used for the refactor.
- Ideally the routes are constant and live outside a react component to
avoid constant recreation of the router/routes
- While you are meant to define routes as code, there is a
"bridge"/helper, [`createRoutesFromElements`](
https://reactrouter.com/6.30.1/utils/create-routes-from-elements), that
translates JSX `<Route>` objects into code. I used that to keep the diff
and changes somewhat manageable. We can switch to a purely code based
definition in the future.
- File-Base/Framework: You can define routes through your hierarchy on
the file system. (not used)
- RR v6 `<Routes>` do not have a `render={() => ...}` prop anymore that
would run any function. That makes it impossible to do any sort of
conditional logic inside the router. I moved all these methods into new
React wrapper components.
- RR v6 uses new hooks: `useNavigation`, `useParams`(for accessing route
parameters, e.g. `/tasks/:taskId`)
- RR v6 allows [nested
routing](https://reactrouter.com/6.30.1/start/overview#nested-routes). A
common parent page (with nice layout) can be used to render/insert the
child views with an `<Outlet />` component. I used it for the orga &
account pages for rendering the various tabs into a comment parent.
Also, the router's root element uses an `<Outlet>`.
- RR v6 does not support class-based React component anymore. The
`withRouter` HOC was removed. I build a new `withRouter` HOC as a
workaround for our remaining React class components.
- RR v6 removed an imperative API for blocking the navigation from one
page to another. It was replace with a hook
[`useBlocker`](https://reactrouter.com/6.30.1/hooks/use-blocker).
Unfortunately, we mostly use the blocker in class based components.
There is not easy workaround - so I had to remove some of the blocking
stuff. (Dataset Setting are currently being refactored as functional
components and will be able to use that once more. #8732 )


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

### Steps to test:
My testing process:
- Click through all the routes/views and make sure they still work
- Make sure that routes with parameters still work, e.g. all the task,
projects, task types stuff with ids and prefilled forms
- Test a legacy route

### TODOs:
- [x] Fix: navigation blockers
- [ ] Decide: Should we rename the `SecuredRoute` component? It is not a
`<Route>` any more... maybe `AuthRequired` or `RequireAuth` or
something.
- [x] Fix: `features` type definition. It needs too load earlier.
- [x] Fix: dashboard header having too many items <img width="1650"
alt="Screenshot 2025-07-03 at 09 37 11"
src="https://github.com/user-attachments/assets/9bba76e3-2b3a-4bad-8be0-798685d0e4ac"
/>


### Issues:
- fixes #

------
(Please delete unneeded items, merge only when none are left open)
- [x] 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

---------

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>
Co-authored-by: Michael Büßemeyer <39529669+MichaelBuessemeyer@users.noreply.github.com>