Polish Notifications Permissions Request

[ovitrif]

Some UX improvements could be addressed in follow-up PRs:

• Request permission once even without LN balance because of CJIT and channel opening, or at least in these specific flows
• Make the FG service optional, since it became less vital after ldk-node caching improvements
  • Could be an option in background payments settings. Would need some design work

Originally posted by @jvsena42 in #1004 (comment)