fix(fullnode): reliable rescan completion + self-explaining shielded send errors

Two related fixes for the post-bootstrap "send fails / rescan stuck at 99%" trap:

1) Rescan completion now keys off warmup-end. A -rescan runs entirely inside daemon
   warmup (every RPC returns -28 until it finishes), so warmup completing IS the rescan
   completing. The old detectors relied on getrescaninfo (which some daemons answer with
   "Method not found") or a "Done rescanning"/bench log line the daemon may never print,
   leaving the status bar stuck at 99% — so users killed the rescan before it finished.
   When warmup ends and a rescan was confirmed active, clear the rescan state, flip to
   100%, refresh history/balance, and toast completion.

2) z_sendmany failures that mean stale shielded note data (shielded-requirements-not-met,
   missing sapling anchor, invalid sapling spend proof, bad-txns-sapling-*) now append a
   plain-language hint telling the user to run a full rescan, instead of surfacing only the
   raw daemon string.

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

src/util/i18n.cpp

@@ -1293,6 +1293,7 @@ void I18n::loadBuiltinEnglish()
         strings_["send_tx_sent"] = "Transaction sent!";
         strings_["send_tx_success"] = "Transaction sent successfully!";
         strings_["send_status_unconfirmed"] = "Transaction status could not be confirmed";
+        strings_["send_err_needs_rescan"] = "Your wallet's shielded note data is out of date with the blockchain (this happens after a bootstrap or reindex). Run a full rescan via Settings -> Rescan Blockchain and let it finish completely, then try sending again.";
         strings_["send_txid_copied"] = "TxID copied to clipboard";
         strings_["send_txid_label"] = "TxID: %s";
         strings_["send_valid_shielded"] = "Valid shielded address";