fix(security): color seed-save by outcome; explain the Encrypt button; friendly key-export errors

- Seed-backup dialog: a failed "Save to file" was drawn in the success (green)
  colour; it's now red on failure, green on success (flag reset on close).
- Encrypt-wallet dialog: the disabled Encrypt button now says why once the user
  starts typing ("Use at least 8 characters." / "The passphrases don't match."),
  and a "Show passphrase" checkbox reveals both fields.
- Key-export dialog: the raw RPC exception (e.g. "...walletpassphrase first")
  is routed through util::friendlyRpcError(), so a locked wallet reads
  "Unlock your wallet first, then try again." instead of daemon jargon.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.h

@@ -1402,6 +1402,7 @@ private:
     // mnemonic) and is wiped with sodium_memzero when the dialog closes.
     std::string seed_backup_phrase_;
     std::string seed_backup_status_;
+    bool        seed_backup_status_ok_ = true;   // colour the status line by outcome (green save vs red fail)
     bool seed_backup_fetch_started_ = false;
     bool seed_backup_loading_ = false;
     bool seed_backup_no_mnemonic_ = false;