fix: compilation errors with Qt and Discord RPC

- Downgrade Qt version from 6.8.1 to 6.7.3 as 6.8.1 is not yet widely available
- Add USE_DISCORD_PRESENCE check around discord-rpc compile options to prevent
  build errors when Discord integration is disabled

CMakeLists.txt

@@ -419,7 +419,7 @@ add_subdirectory(externals)
 
 if (ENABLE_QT)
     if (NOT USE_SYSTEM_QT)
-        download_qt(6.8.1)
+        download_qt(6.7.3)
     endif()
 
     find_package(Qt6 REQUIRED COMPONENTS Widgets Multimedia Concurrent Network)
@@ -674,4 +674,6 @@ if(ENABLE_QT AND UNIX AND NOT APPLE)
             DESTINATION "share/metainfo")
 endif()
 
-target_compile_options(discord-rpc PRIVATE -fpermissive)
+if (USE_DISCORD_PRESENCE)
+    target_compile_options(discord-rpc PRIVATE -fpermissive)
+endif()