Add Dynamic Discord RPC Support

[zydezu]

Optional Discord status showing the current stage, whether you're playing as Sonic or the Werehog, and elapsed time. Controlled by a new [System] DiscordRichPresence option (on by default, toggleable at runtime).

• Talks to the local discord-ipc socket directly — no SDK or new dependency (reuses the vendored nlohmann/json). Works on Windows, Linux and macOS, including Flatpak/Snap Discord.
• Runs on its own worker thread with reconnect/backoff, so a closed or missing Discord never touches the game loop. Presence is throttled and only resent when it changes.
• Stage and status text goes through Localise(); keys are in locale.cpp, English-only for now with the normal fallback until translated.
• large_image/small_image map to per-region and Sonic/Werehog art-asset keys, left empty (app icon) when no matching asset is uploaded.
• Flatpak manifest gains --filesystem=xdg-run/discord-ipc-0 for socket access.