feat(presets): generalize Flatpak/native detection for builtin launch presets

[hikaps]

Summary

• Generalize Flatpak vs native binary detection across all builtin launch presets (Steam, Heroic, Lutris), replacing the previous Heroic-only detection with a unified resolveLaunchCommand() helper in PresetManager
• Add flatpakAppId and flatpakArgs fields to the LaunchPreset struct so each preset declares its Flatpak alternative, with full serialization and QML exposure
• Cache resolved commands to ~/.config/couchplayrc with a 24h TTL to avoid repeated which/flatpak subprocess calls at startup
• Add DualScope credit to README

Context

Previously only Heroic had Flatpak awareness for its launch command. Steam and Lutris presets hardcoded native commands (steam -tenfoot -steamdeck, lutris), which would fail on systems where only the Flatpak version is installed (common on Bazzite/immutable distros).

Changes

Preset	Native command	Flatpak fallback
Steam	steam -tenfoot -steamdeck	flatpak run com.valvesoftware.Steam -tenfoot -steamdeck
Heroic	heroic	flatpak run com.heroicgameslauncher.hgl
Lutris	lutris	flatpak run net.lutris.Lutris

Resolution priority: native first → Flatpak fallback → native as-is (graceful degradation).

Untouched

• HeroicConfigManager config path detection and isFlatpak() display flag
• SteamConfigManager config sync paths
• CommandVerifier implementation
• QML display logic