Add cross-platform settings and application shell

[whatever-industries]

Summary

• add a persistent SDL3 and Dear ImGui application shell with one configured external asset root
• automatically launch a valid saved root and show only a compact configuration prompt when setup is missing or the game is stopped
• add a separate settings window for game assets, display, audio, subtitles, keyboard, and gamepad controls
• launch automatically after successful asset selection, revalidation, or disc import; restart an active session when its root changes
• provide native macOS Settings, Controls, and Stop Game menu commands, plus an Open Settings button and F10/controller access on Windows and Linux
• support explicit data roots, direct GAGBoy launch, aspect-preserving window scaling, and clean engine-session teardown
• package native Windows, macOS, and Linux metadata and multi-resolution application icons
• pin frontend dependencies and extend release-package audits and documentation

Compatibility

• keeps existing CDF, SFS, save, and freegag.ini formats
• keeps user-supplied game data external and never downloads or deletes it
• preserves the existing SDL3 renderer and extractor CLI
• migrates older frontend preferences by retaining only the previously selected asset root

Validation

• macOS Debug build
• C++ formatting checks
• GitHub Actions workflow parse
• macOS plist, bundle-resource, and Dock icon checks
• ICNS, ICO, and Linux icon-resolution validation
• CPack archive-content check
• normal configured-root auto-launch
• native Stop Game transition to the compact configuration prompt
• Settings access from the prompt and macOS application menu
• manual game, settings, controls, resize, and relaunch testing with prepared GAG assets
• git diff whitespace check

The repository currently defines no CTest targets.

[coderabbitai]

Important

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⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Team

Run ID: c8bc9838-5af1-4128-9424-fe2935b7b4c4

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[superg]

I reimplemented timer fix and downloaded high res icon for a later inclusion, the other changes are discarded.