Closes#1195.
Adds `surfsense_web/app/dashboard/error.tsx` so unhandled errors in any
dashboard route render inside the dashboard layout (sidebar/nav stay
visible) instead of blowing out to the root-level error page. `reset()`
now retries just the failed segment.
Follows the same PostHog capture pattern as the root error.tsx and adds
a secondary "Go to dashboard home" link for recovery when `reset()`
doesn't help.
