Add the launcher UI to Linux builds

[MysteriousAeon]

Fixes #3.

Linux releases currently launch nds_runner directly, so the recomp-ui launcher and its configuration pages are unavailable.

This adds native Linux support to the existing launcher and packages it into the AppImage:

• launches nds_runner with posix_spawn on Linux while preserving the existing Windows path
• persists launcher settings under the XDG config directory
• separates the read-only AppImage runtime directory from writable per-install data beside the AppImage
• makes the launcher the default AppImage entry point while preserving explicit runner arguments
• packages the launcher assets and dependencies
• extends the AppImage layout test to verify the launcher and assets are present

Verified locally:

• native Linux launcher builds and opens correctly
• verified ROM launches successfully through the UI
• launcher settings and generated BIOS identity persist outside the AppImage
• full Linux AppImage build succeeds
• AppImage layout test passes with a read-only AppDir
• launcher and runner exit cleanly after gameplay
• Windows launcher cross-build succeeds with MinGW

Note: this branch also contains the one-line FMV bank verification fix from #10 because the current main build otherwise false-fails under pipefail. If #10 lands first, that overlap should disappear on rebase.