fix(wallet): surface the real z_getnewaddress error; sweep coinbase in migrate-to-seed

From a legacy-wallet migration support case (mined T-funds, 3y-old Silent Dragon wallet.dat): the balance
shows but new-address and migrate both fail with unhelpful/silent results. Two fixes:

1) Real address-creation error. createNewAddress() swallowed the daemon's z_getnewaddress error and the UI
   showed a generic "Failed to create shielded address". It now keeps the daemon's reason (friendly-mapped
   via friendlyRpcError) in last_address_error_, which the Receive tab and request-payment dialog show
   instead of the generic string — so a legacy/degraded wallet sees the actual "HD seed not found". And when
   that specific error appears it flips wallet_degraded_ + warns once, even for an EXTERNAL daemon
   (detectWalletDegraded() only scans the embedded daemon's log and would otherwise miss it).

2) Migrate-to-seed sweep skipped coinbase. beginSweepToSeedWallet()'s z_mergetoaddress ignores coinbase
   UTXOs, so a miner's coinbase-only balance produced "Could not find any funds to merge", the legacy
   balance never reached ~0, and the adopt gate stalled silently. The sweep now falls back to
   z_shieldcoinbase("*", dest, fee, 50) to the SAME shielded destination when the merge reports no funds.
   z_shieldcoinbase returns an opid just like the merge, so the single-opid adopt gate (opid -> txid ->
   confirm -> balance ~0 -> adopt) is unchanged, and a batch limit / mixed wallet drains via the existing
   "Sweep remaining" re-sweep.

Fund-path: the z_mergetoaddress call and its args are byte-identical; the fallback only ADDS a
z_shieldcoinbase to the same dest when merge finds nothing, and nothing is adopted until the swept funds
confirm and the legacy balance is ~0 (gate unchanged). Runtime-verified behavior (clean build + ctest; the
RPC paths are not pure-testable).

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

src/app.h

@@ -453,6 +453,9 @@ public:
     // Address operations
     void createNewZAddress(std::function<void(const std::string&)> callback = nullptr);
     void createNewTAddress(std::function<void(const std::string&)> callback = nullptr);
+    // The daemon's actual reason the last new-address RPC failed (friendly-mapped), or "" on success. Lets
+    // the UI show e.g. "HD seed not found" (legacy/degraded wallet) instead of a generic "couldn't create".
+    const std::string& lastAddressError() const { return last_address_error_; }
 
     // Hide/unhide addresses from the address list (persisted in settings)
     void hideAddress(const std::string& addr);
@@ -1350,6 +1353,7 @@ private:
     // so a possibly-incomplete salvaged wallet isn't mistaken for fund loss. Warned once per session.
     bool wallet_auto_recovered_ = false;         // a salvage happened this session
     bool wallet_auto_recovered_warned_ = false;  // guard: only surface it once per session
+    std::string last_address_error_;             // daemon reason the last z_getnewaddress/getnewaddress failed ("" = ok)
     bool wallet_degraded_ = false;               // v1.3.0+ opened the wallet in DEGRADED mode (no new HD keys)
     bool wallet_degraded_warned_ = false;        // guard: surface the degraded-mode notice once per session
     bool show_wallet_recovered_dialog_ = false;  // auto-shown warning dialog