Add OverlayScrollArea with shared thumb metrics and drag handling; size the
thumb against the rail track height so panel insets cannot push it past the
visible rail. Route scroll uses a stable viewport id; Genres restores scroll
and infinite-scroll observation against that viewport (merged with upstream
virtualized genres list and lazy routes behind Suspense).
Suppress queue resizer activation when the pointer targets the main-route
overlay scrollbar; disable the resizer while dragging the thumb and use a
grabbing cursor on the body.
Apply WebKitGTK smooth wheel to the mini webview as well as main; the mini
window reapplies the persisted setting after auth store hydration.
cucadmuh