Implement Windows DLL blocklist to prevent problematic injections

[Copilot]

Astrill VPN's ASProxy64.dll injection causes access violations in RustDesk. Implementing blocklist following OBS Studio's approach.

Implementation

src/platform/win_dll_blocklist.c - Core blocking logic with dual modes:

• Without Detours (default): SetDefaultDllDirectories restricts DLL search to System32, app dir, user dirs
• With Detours (optional): Hooks NtMapViewOfSection to intercept and block DLL loads

Blocklist includes:

• asproxy64.dll (Astrill VPN - primary target)
• 17 additional DLLs from OBS Studio's proven list (RTSS, FRAPS, Bandicam, etc.)

Integration

Initialization added to all Windows entry points:

• main.rs - All 3 main() variants
• service.rs - Windows service
• flutter_ffi.rs - Flutter interface

#[cfg(windows)]
crate::platform::init_dll_blocklist();

build.rs compiles the C file via cc crate. Logs to %TEMP%\rustdesk_dll_blocklist.log.

Optional Detours support via res/vcpkg/detours/vcpkg.json.

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