Games browser: hint when content exists but no core can run it

Surfaced by a games-DB design review (adversarially verified against source).

The Games menu entry is now hidden unless the index has games (empty page removed, by design). But indexing is gated by MenuGamesCoreAvailable — with zero cores installed (or no core matching the ROMs present), a user with a full content directory ends up with the Games entry simply absent and no explanation. About ▸ Available Cores shows "(none found)", but that is easy to miss.

Approach (respecting the hidden-entry design). When a content dir is set and nothing got indexed because no installed core matched, surface a hint — e.g. a one-shot toast after the scan ("No installed core can run the content found — add a core, then Rescan"), or a line under Settings ▸ Directories. Optionally add a games-layer counter of files rejected purely by the core-availability gate (incremented only when the hook was non-NULL and returned false) so the message can distinguish "no cores at all" from "cores present but none match". Low severity / UX.

Start by tracing the Games menu indexing and the MenuGamesCoreAvailable gate during the content scan. Check the existing About ▸ Available Cores and Settings ▸ Directories behavior, then define the hint's trigger and wording for content rejected because no installed core matches, while keeping the empty Games entry hidden.