frontend,libobs: Add Program Files plugin path and module dedup check

[marcusbooker77]

Summary

Two small improvements to the Windows plugin loading system:

1. Add Program Files plugin search path (OBSBasic.cpp)

Adds C:\Program Files\obs-studio\obs-plugins\64bit\ as an additional module search path in AddExtraModulePaths(). This allows development/custom builds of OBS to discover and load third-party plugins that were installed by the standard OBS installer into Program Files.

Use case: When building OBS from source for development or testing, third-party plugins (StreamFX, Move Transition, etc.) installed via the official installer are invisible to the dev build. This change makes them available without manual copying.

#if defined(_WIN32)
    {
        char pf[512];
        DWORD pf_len = GetEnvironmentVariableA("ProgramFiles", pf, sizeof(pf));
        if (pf_len > 0 && pf_len < sizeof(pf)) {
            string std_bin  = string(pf) + "/obs-studio/obs-plugins/64bit";
            string std_data = string(pf) + "/obs-studio/data/obs-plugins/%module%";
            obs_add_module_path(std_bin.c_str(), std_data.c_str());
        }
    }
#endif

2. Module deduplication check (obs-module.c)

Adds a check in load_all_callback() to skip modules that have already been loaded from a higher-priority search path. Without this, the same plugin DLL found in multiple search paths would be loaded twice, causing duplicate sources/filters in the UI and potential memory/stability issues.

/* Skip modules already loaded from a higher-priority path. */
if (obs_get_module(info->name)) {
    blog(LOG_DEBUG, "Skipping module '%s' (%s), already loaded",
         info->name, info->bin_path);
    return;
}

Test Plan

• Dev build discovers plugins from C:\Program Files\obs-studio\obs-plugins\64bit\
• Plugins appear only once in the source/filter lists (no duplicates)
• LOG_DEBUG messages confirm dedup is working: "Skipping module 'xxx', already loaded"
• Portable mode still works (early return in AddExtraModulePaths before the new path)
• No regressions on Linux/macOS (changes are #if defined(_WIN32) guarded)

Files Changed

File	Change
frontend/widgets/OBSBasic.cpp	Add Program Files search path in AddExtraModulePaths()
libobs/obs-module.c	Add dedup check in load_all_callback()

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