feat(core): fold group side nav overflow into a height-based show-more

[antfubot]

Description

When a group has more members than the panel is tall enough to show, the group side nav (DockGroupSidebar) previously just scrolled. This folds the members that don't fit into a bottom "show more" button that opens them in a flyout, driven by the rail's measured frame height — so the rail never scrolls.

• Height-based capacity. The rail root is measured with useElementBounding (mirroring DockPanel). A new pure deriveSidebarCapacity helper turns the available height into an integer capacity, then feeds the existing docksSplitGroupsWithCapacity split. It reserves the show-more button's slot only once overflow is unavoidable (two-pass), and budgets for sub-category dividers so a multi-sub-category group never clips.
• Reused components. The flyout reuses DockGroupPopover (passed only the overflowed members), opened through a dedicated docksSidebarOverflowPanel floating slot (placement right) so it stays independent of the dock bar's group/overflow popovers. The trigger matches the dock bar's DockOverflowButton — the same dots icon and a 9+-capped count badge.
• Hidden selection stays visible. When the active tool is one of the hidden members, the show-more button lights up (also while the flyout is open), so a hidden selection remains discoverable.
• The rail switches from of-y-auto to of-y-hidden; all three hosts (DockPanel, DockStandalone, DockEdge) inherit the behavior.

Linked Issues

Additional context

• Unit tests cover the capacity derivation and the overflow split (dock-sidebar-capacity.test.ts).
• An Overflow Storybook story on a constrained-height shell demonstrates the folded rail with a highlighted show-more button.
• pnpm lint, pnpm test, pnpm typecheck, and pnpm build all pass. The .generated/css.ts change is the UnoCSS artifact regenerated from the new template classes.

This PR was created with the help of an agent.