Refactor desktop startup/shutdown ownership paths and remove duplicated UI validation logic

[Copilot]

This change targets code quality debt in the desktop entrypoint by removing manual lifetime management and duplicated logic in high-traffic initialization/UI code. It keeps behavior intact while making resource handling and argument conversion safer and easier to reason about.

• Ownership and lifetime cleanup (RAII)

  • Replaced raw-pointer ownership in src_desktop/main.cpp with std::unique_ptr for app-level objects:
    • SingleInstanceManager
    • WebsocketClient
  • Removed manual new/delete cleanup paths tied to app exit.

• Windows argument conversion hardening

  • Reworked wWinMain UTF-16 → UTF-8 argv construction to use RAII containers (std::vector<std::string>, std::vector<char*>) instead of manual heap buffers.
  • Corrected conversion buffer sizing/resize flow to avoid null-terminator sizing pitfalls.

• UI validation deduplication

  • Consolidated repeated “draw invalid field border” ImGui logic into a local helper used by hostname/port/FPS validation, reducing repeated rendering code.

• Scope cleanup

  • Reverted an unrelated react-web/package.json version bump to keep this PR focused on the desktop refactor.

// Before: manual ownership
SingleInstanceManager* instanceManager = new SingleInstanceManager(...);
...
delete instanceManager;

// After: RAII ownership
std::unique_ptr<SingleInstanceManager> instanceManager =
    std::make_unique<SingleInstanceManager>(...);