feat: Gray out disabled chapters, show DISABLED tooltip

[EttyKitty]

Disabled chapters draw icons at 0.5 alpha and replace hover description with DISABLED.

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Summary by cubic

Disabled chapters now render at 50% opacity and show a "DISABLED" tooltip on hover instead of their usual description, so locked chapters are immediately identifiable in the creation menu.

^{Written for commit 4cdeabc. Summary will update on new commits.}

[EttyKitty]

/oc Address these points (implement, unless they introduce regressions and I'm wrong): 1. Unless I'm wrong, I think slate4 is never mutated from the scope of draw_chapter_select function. As such, I think slate4 / 30 always produces the same value, yet the op is now repeated 14 times in the file. I don't think that's needed, and the result can likely be cached in a local var at the earliest point, then reused. 2. chap.disabled ? 0.5 : 1 is now copy-pasted 4 times. Again, can likely be cached to a local var at the earliest point and reused.

[lyra-cactus]

Changes pushed to branch automation/dispatch-1791494652.

[EttyKitty]

/review

[lyra-cactus]

LGTM - small, focused change. Alpha caching (_base_alpha) and static DISABLED_ALPHA clean up repeated slate4 / 30 math, icon dimming is correctly scoped with restore after each draw, and click guards already prevent selecting disabled chapters. One i18n nit below.