Show errors for missing or unsupported ROMs

[MysteriousAeon]

Fixes #6.

This adds visible ROM validation errors instead of silently exiting when the expected ROM is missing or unsupported.

Changes:

• Windows launcher now shows an error when no ROM is selected, the ROM cannot be opened, or its SHA-1 does not match the supported USA revision 0 dump.
• Linux AppImage startup now shows a desktop error dialog when no .nds file is found or when the detected ROM has the wrong SHA-1.
• Explicit runner arguments on Linux remain unchanged.

Tested on Linux AppImage:

• No ROM: error dialog shown.
• Wrong ROM: unsupported-ROM error dialog shown.
• Correct ROM: game launches normally.

git diff --check passes and the change is limited to launcher/recomp-ui/launcher_main.cpp and tools/build-linux.sh.