mgr/dashboard: Fix username validation for special characters by URL-encoding user...

mgr/dashboard: Fix username validation for special characters by URL-encoding user lookup requests

When validating usernames, special characters such as '#' were not being URL-encoded before being appended to the user lookup endpoint.
Because the browser treats '#' as a URL fragment, the request path was truncated, causing the backend to return an incorrect response and the UI to display a misleading "User already exists" validation error.

Fixes: https://tracker.ceph.com/issues/77129
Signed-off-by: Aashish Sharma <aasharma@redhat.com>
(cherry picked from commit b2ed52b2a4d86ad6fa19851053badfa9118c4b6f)