Fixed broken mentor links and a "-+" glitch on the homepage

Two small but visible trust issues on the homepage: the circular mentor-avatar collage and the "Meet some of our Mentors" grid could link to `/mentors/undefined` instead of a real profile — either during the moment mentor data was still loading, or for the rare mentor record missing a username. Those spots now simply skip the broken link instead of showing one.

The "we already have over N+ mentors" line had a similar issue: while the mentor count was still loading (or if that request ever failed), it rendered a literal "-+" instead of a number, which read as an obviously broken page rather than a normal loading state. The sentence now waits for a real count before showing the "over N+" part at all, so nothing implies a number that isn't real.

This is the first entry in a broader pass through the platform's public-facing trust and reliability issues — broken links, stale copy, unverified public counts, and similar rough edges that make the site feel unmaintained even when the underlying product works.