Inventory UI fixes

[BenjaminAmos]

Description

This pull request fixes various issues encountered when testing the inventory UI a bit more thoroughly:

• Fixes a crash when dropping the player's shield.
• Fixes a crash when using keyboard navigation after all items are sold.
• Changes highlight behaviour to target the nearest item after the currently highlighted item is removed.

Testing

• Start a new game
• Open the inventory screen and drop each of your items one-by-one.
• When an item is dropped, the highlight cursor should move to the next item along (or the previous item, for the last item).
• The game should not crash when you drop your shield.
• Additionally, follow the instructions in list highlight crashing program #711 and ensure that the game does not crash.

Notes

This fixes #711.

[NicholasBatesNZ]

Thanks for digging into these — the item == null guard in SolShip.maybeUnequip looks right and fixes a real latent NPE. I think the selection rework needs another pass, though.

In updateItemRows() the new fallback reads items.groupCount() < selectedIndex ? items.getGroup(selectedIndex) : null — the comparison looks inverted (it calls getGroup precisely when the index is out of range) and it's missing the page * Const.ITEM_GROUPS_PER_PAGE offset that every other lookup uses, so it can throw the very IndexOutOfBoundsException it's meant to prevent.

Relatedly, the items.groupCount() > 0 guards in onKeyEvent don't cover #711's actual trace (Index: 7, Size: 7 — the container wasn't empty, selectedIndex was just stale), and since setSelected no longer falls back to selectedIndex = 0, nothing clamps selectedIndex after a removal on page 0 any more; guarding on selectedIndex + page * Const.ITEM_GROUPS_PER_PAGE < items.groupCount() would handle both. That may be what @dreaddymck is still hitting on the PR build.

One smaller thing: in getSelectionAfterRemove, idx <= 0 now conflates "not found" (-1) with "the first group" (0), so removing the top item returns no selection instead of the next one — idx < 0 looks like what you want.

Good news is it still merges cleanly onto develop after the Gradle 9.6.1 / Java 17 bump, so no rebase needed.