fix(repo-appearance): tint active-tab highlight in spine with repo color
`ShelfSpineTabSlot.backgroundFill` still hardcoded `Color.accentColor`
for the active-tab fill, so a repo with a pinned color showed a
spine background in that color but its selected tab indicator in the
default accent — the two clashed.
Thread the spine's `effectiveTintColor` (repo color, or accentColor
fallback) into each slot via a new `activeHighlightTint` parameter
and use it for the active fill. The proximity-driven
`activeHighlightAlpha` ladder is unchanged so the selection
indicator still fades with distance from the open book; only the
hue swaps to keep the slot in the same color family as the spine.
Notification tint stays orange — it's an attention idiom that has to
read regardless of repo color.