When manually adding a single game, allow platform selection from the supported platform list

Description:
When manually adding a single game, the Platform field currently behaves as a freeform/editable field that only suggests platform names already present in the user’s library. If the desired platform does not already exist, it must be entered manually.

I would like to request that this field use LaunchBox’s supported/canonical platform list instead, so users can select the platform from a standardized picker when adding a single game.

Why this would help:

• Reduces typos and naming inconsistencies

• Makes manual game entry faster and easier

• Helps keep platform names aligned with LaunchBox’s supported platforms

• Improves metadata matching and emulator/platform association reliability

Current behavior:
When importing or manually adding a single game, the platform must be typed manually unless that exact platform name already exists in the library.

Requested behavior:
Provide a selectable drop-down or searchable picker populated from the supported platform list, while still optionally allowing custom manual entry if needed.

That would make the single-game add flow more consistent with the rest of the import experience and easier for users who are adding games one at a time.