fix(history): show shielding moves as neutral "Shielded", not red "Sent"

A coinbase/transparent→shielded move (often auto-shield) was drawn with a red
"Sent" label and send arrow yet a green "+X" amount — an internally
contradictory row that reads as "money leaving" to anyone who auto-shielded. It
now renders as a neutral "Shielded" row (shield icon + the shielded accent
colour for label and amount), and since a shield is your own funds going
private — not a spend — it's excluded from the Sent card total and the Sent
filter (it merges into one row visible under "All").

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

src/util/i18n.cpp

@@ -1207,6 +1207,7 @@ void I18n::loadBuiltinEnglish()
         strings_["tx_chat_badge"] = "Message";
         strings_["recv_type"] = "Recv";
         strings_["sent_type"] = "Sent";
+        strings_["shielded_self_type"] = "Shielded";
         strings_["immature_type"] = "Immature";
         strings_["mined_type"] = "Mined";
         strings_["mature"] = "Mature";