fix(send): validate recipient address checksums (Base58Check + Bech32)

The send screen labelled any prefix+length match as a "Valid" address, so a
mistyped address that still matched the pattern passed the gate. Add pure,
offline checksum validation — Base58Check (transparent R-addresses) and Bech32
(Sapling zs-addresses) — and require it in the validity check. Both verifiers are
version-byte/HRP agnostic (the HRP is taken from the string, the Base58 checksum
is chain-independent), so a correct implementation never rejects a genuine
address while catching transcription errors. Works for both build variants
(no daemon round-trip), unit-tested against standard BIP173 / Base58Check vectors.

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

CMakeLists.txt

@@ -472,6 +472,7 @@ set(APP_SOURCES
     src/util/logger.cpp
     src/util/async_task_manager.cpp
     src/util/amount_format.cpp
+    src/util/address_validation.cpp
     src/util/base64.cpp
     src/util/single_instance.cpp
     src/util/i18n.cpp
@@ -980,6 +981,7 @@ if(BUILD_TESTING)
         src/ui/windows/mining_tab_helpers.cpp
         src/util/payment_uri.cpp
         src/util/amount_format.cpp
+        src/util/address_validation.cpp
         src/data/wallet_state.cpp
         src/data/transaction_history_cache.cpp
         src/daemon/lifecycle_adapters.cpp