amuleapi: expose a guest (low-rights) password like the web server? #593

Summary

The Preferences dialog lets you configure two credentials for the classic
Web server: a Full rights password and an optional Low rights (guest)
password. For amuleapi it only exposes a single Admin password. This is
a suggestion to discuss whether amuleapi should get guest parity — no strong
opinion, just raising it so the team can decide.

Current behaviour

Web server — both passwords are stored (encrypted) in amule.conf:

• Full rights → /WebServer/Password
• Low rights / guest → /WebServer/PasswordLow
• Enable low-rights user → /WebServer/UseLowRightsUser

amuleapi — only the admin password is exposed:

• Admin → /AmuleApi/Password in amule.conf
• When amule launches amuleapi (--amule-config-file), this value is applied
  only as the admin password, in memory. The amuleapi-passwords file on
  disk is not touched.

amuleapi does support a guest role internally (read-only), but its password
can currently only be set via the CLI:

amuleapi --set-guest-pass=<plain>

which writes it to the amuleapi-passwords file. There is no field for it in
the Preferences dialog, and amule never pushes a guest password when it starts
amuleapi.

Questions for discussion

1. Should the Preferences dialog expose an amuleapi guest (low-rights)
   password too, mirroring the web server (a /AmuleApi/PasswordLow field
   plus an enable checkbox), so amule pushes it to amuleapi on startup?
2. If added, should these amuleapi credentials be persisted to disk when
   changed (e.g. written to amuleapi-passwords), or kept in-memory only like
   the current admin push — so a standalone operator's CLI-set passwords are
   preserved?

No preference here — flagging the asymmetry and letting you decide whether it's
worth it or working as intended.