Improve startup performance, shutdown reliability, and Qt5 compatibility

- Defer SecureMsgStart and ReacceptWalletTransactions to background thread
  so GUI appears faster during startup
- Add shutdown checks in wallet scan/reaccept to prevent hanging on exit
- Smart wallet rescan: start from wallet birthday instead of genesis block
- Raise default dbcache from 25 to 128 MB for better performance
- Replace deprecated QDesktopServices with QStandardPaths (Qt5 compat)
- Enforce Qt5+ requirement in build system, remove -fpermissive flag

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

triangles-qt.pro

@@ -7,13 +7,13 @@ DEFINES += QT_GUI BOOST_THREAD_USE_LIB BOOST_SPIRIT_THREADSAFE BOOST_THREAD_PROV
 CONFIG += no_include_pwd
 CONFIG += thread
 
-QMAKE_CXXFLAGS = -fpermissive
-
-greaterThan(QT_MAJOR_VERSION, 4) {
-    QT += widgets
-    DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0
+lessThan(QT_MAJOR_VERSION, 5) {
+    error(Triangles Qt requires Qt 5 or newer)
 }
 
+QT += widgets
+DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0
+
 # UNCOMMENT THIS SECTION TO BUILD ON WINDOWS
 # MSYS2 MinGW64 paths
 win32 {