Add MCP theme export

[Quantumyilmaz]

Summary

• Adds an Export as Theme workflow to the MCP Theme page.
• Exposes every exportable theme field through MCP, including metadata and advanced special-effect parameters.
• Exports the live Theme::default_theme values as a formatted JSON theme.
• Registers a successful export immediately, so clients can request it without using Reload Themes first.

Export behavior

• Proposes the first available MyThemeN filename, starting with MyTheme1.
• Lets the user edit the name and automatically appends .json.
• Rejects invalid Windows filenames, path separators, reserved names, and case-insensitive collisions.
• Disables Export while the name is invalid or already exists and never overwrites an existing file.
• Shows the saved path after a successful export.
• Deliberately omits hide_in_menu and n_max_buttons from exported themes.

Implementation notes

• Persists the newly editable metadata and special-effect values in MCP settings.
• Adds localized English strings for the new controls and export modal.
• Moves the theme-registry lock ahead of lookup in ProcessRequestTheme so runtime registration and requests use the same synchronization.

Verification

• Debug configuration generated successfully with MSVC/CMake/Ninja.
• Full Debug compile and link succeeded.
• git diff --check passed.
• English translation file remains UTF-16LE with CRLF; exactly the intended 20 rows were added and match the C++ fallbacks.
• In-game MCP export smoke test.