feat(shield): pick the mining address from a dropdown + show a confirm preview

Shield-coinbase replaced its free-text "*"-wildcard address box with a dropdown
of the wallet's mining (transparent) addresses and their balances ("All mining
addresses" default), and the confirm step now previews "Shield up to X DRGX from
N mining address(es)… to <label>? Network fee: Y" instead of only relabeling the
button. Also blocks submitting from an empty source or one with no funds. The
z_shieldcoinbase call (args/fee/limit/destination) is unchanged.

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

src/util/i18n.cpp

@@ -2621,7 +2621,9 @@ void I18n::loadBuiltinEnglish()
         strings_["shield_check_status"] = "Check Status";
         strings_["shield_completed"] = "Operation completed successfully!";
         strings_["shield_description"] = "Shield your mining rewards by sending coinbase outputs from transparent addresses to a shielded address. This improves privacy by hiding your mining income.";
-        strings_["shield_from_address"] = "From Address:";
+        strings_["shield_from_address"] = "From mining address:";
+        strings_["shield_all_mining_addrs"] = "All mining addresses";
+        strings_["shield_confirm_fmt"] = "Shield up to %s DRGX from %d mining address(es) (at most %d rewards per run) to %s? Network fee: %s DRGX.";
         strings_["shield_funds"] = "Shield Funds";
         strings_["shield_in_progress"] = "Operation in progress...";
         strings_["shield_max_utxos"] = "Max UTXOs per operation";