Fix touch device mod getting selected as a free mod in playlists

[frenzibyte]

• Closes Freemods incorrect mod count after deselecting all mods #26490

The cause of this issue laid in the way FooterButtonFreeMods determines available mods, which is the following code:

osu/osu.Game/Screens/OnlinePlay/FooterButtonFreeMods.cs

Lines 142 to 144 in 8e5b2e7

	private IEnumerable<Mod> allAvailableAndValidMods => freeModSelectOverlay.AllAvailableMods
	.Where(state => state.ValidForSelection.Value)
	.Select(state => state.Mod);

Touch device has ValidForSelection = true, therefore being included in the above list and becoming selected when clicking the footer button.

At first, I considered removing system mods from being added to ModSelectOverlay.AvailableMods, but I'm not confident enough that this won't indirectly result in random issues when interacting with the mod select overlay (causing the touch device mod to be unintentionally deselected).

Instead, I settled down on setting ValidForSelection = false and using that as an indicator for both FooterButtonFreeMods and SelectAllModsButton (select all button in the overlay) to ignore such mods.

@bdach requesting your review as you're more knowledgable in this system. also worthy of note I cannot test on a touch device at the moment.

[bdach]

seems fine