Improve simulator stability, navigation, and CPU usage

[oldgamer6753]

Summary

This PR includes three small improvements to MarlinSimUI:

• Fix loading of the external imgui.ini file to prevent crashes on subsequent simulator launches.
• Increase mouse wheel zoom speed in the 3D visualization.
• Reduce CPU usage caused by aggressive polling in RawSocketSerial.

Changes

• resources.cpp

  • Replace reserve() with resize() before reading file data.
  • Explicitly null-terminate the buffer.
  • Construct std::string_view with the actual file size.

• visualisation.cpp

  • Increase mouse wheel zoom step and remove frame-time dependency.

• RawSocketSerial.h

  • Increase idle socket polling delay from 1 ns to 1 ms.

Testing

Tested on Windows 10 with Marlin Simulator built through PlatformIO.

Verified:

• Repeated simulator launches with generated imgui.ini no longer crash;
• Mouse wheel zoom is faster and easier to use;
• On an Intel Core i5-6500 3.20 GHz, typical CPU usage dropped from about 59% to about 30%. Results may vary on other hardware;
• Simulator UI and LCD interaction remain responsive.