doctor/home: stale "Online" false-green + duplicated hint literal (minor)

[LukasWodka]

Part of epic tracebloc/backend#1142 (Theme B, cli side + minor).

LOW. Two minor messaging issues:

1. Home "· Online" can be a false green — it reads the backend's stored client status which is up to ~30 min stale (home.go:541-561; backend metaApi/tasks.py:784). Consider "· Online (as of Xm ago)" or a freshness guard.
2. checkReachable strips discover.go's "Diagnose with tracebloc doctor." hint via a duplicated string literal with no test (doctor.go:142) — silently breaks if either string changes. Add a shared const + a test.

[LukasWodka]

Folded into the doctor redesign #365 (reworks doctor/home messaging). Points carried there: (1) home Online up to ~30 min stale — add a freshness guard; (2) checkReachable dup-hint literal.