fix(export): write the transaction CSV atomically and report failures

The transaction export streamed rows straight into an ofstream. A write
that failed partway, on a full disk or removable media, left a truncated
CSV on disk behind a success toast. The CSV formatting also lived inline
in the dialog with no test coverage.

- The CSV is now built in memory and written atomically (temp file plus
  rename), so a write failure is reported with an error toast and leaves
  no partial file.
- Formatting moves into a reusable helper, and the timestamp conversions
  use the thread-safe localtime variants and tolerate a conversion
  failure rather than dereferencing a null result.

Tests: field escaping neutralises formula-injection prefixes and quotes
special characters, and the full CSV has the expected header and rows.

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

tests/test_phase4.cpp

@@ -14,6 +14,7 @@
 #endif
 #include "data/exchange_info.h"
 #include "util/text_format.h"
+#include "data/transaction_csv.h"
 #include "chat/chat_crypto.h"
 #include "chat/chat_identity.h"
 #include "chat/chat_service.h"
@@ -18129,6 +18130,20 @@ void testContactsTextHelpers() {
     EXPECT_EQ(dragonx::util::trimmedClipboardText(std::string(1024, 'a').c_str()).size(), (size_t)1024);
 }
 
+void testTransactionCsv() {
+    std::vector<dragonx::TransactionInfo> txs(2);
+    txs[0].type = "receive"; txs[0].amount = 1.5; txs[0].address = "zs1x"; txs[0].txid = "ab"; txs[0].confirmations = 7;
+    txs[0].timestamp = 1700000000; txs[0].memo = "=HYPERLINK(\"x\")";
+    txs[1].type = "send"; txs[1].amount = -0.00000001; txs[1].memo = "a,b\"c";
+    const std::string csv = dragonx::data::formatTransactionsCsv(txs);
+    EXPECT_TRUE(csv.rfind("Date,Type,Amount,Address,TXID,Confirmations,Memo\n", 0) == 0);
+    EXPECT_TRUE(csv.find(",1.50000000,zs1x,ab,7,\"'=HYPERLINK(\"\"x\"\")\"\n") != std::string::npos);
+    EXPECT_TRUE(csv.find(",-0.00000001,") != std::string::npos);
+    EXPECT_TRUE(csv.find("\"a,b\"\"c\"\n") != std::string::npos);
+    EXPECT_EQ(dragonx::data::escapeCsvField("+1"), std::string("'+1"));
+    EXPECT_EQ(dragonx::data::escapeCsvField("plain"), std::string("plain"));
+}
+
 }  // namespace
 
 int main()
@@ -18438,6 +18453,7 @@ int main()
     testWriteFileIfDifferent();
     testCaptureExtent();
     testResolveExchangeSelection();
+    testTransactionCsv();
 
     if (g_failures != 0) {
         std::cerr << g_failures << " test assertion(s) failed\n";