fix(core): render single overflow dock entry inline instead of behind overflow button

[antfubot]

Description

The dock bar shows a DockOverflowButton (a badge + popover) once entries exceed maxVisibleItems. Hiding a single overflowing entry behind a whole button+popover is unnecessary overhead, so it now renders inline instead.

This is implemented at the split-algorithm level rather than in the component template: docksSplitGroupsWithCapacity gains an opt-in foldSingleOverflow option — when exactly one entry would overflow, it's folded back into visible (in the slot the overflow affordance would have occupied) instead of being handed to the caller as overflow. Dock.vue opts into it for the bar; DockGroupSidebar.vue's own "show more" rail button is untouched (default behavior unchanged) since it has different overflow-affordance semantics. DockOverflowButton still renders as before once two or more entries overflow.

Added unit tests for the new option in dock-sidebar-capacity.test.ts and a SingleOverflowItem Storybook story (Dock.stories.ts) demonstrating the inline behavior.

Linked Issues

Additional context

None.

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commit: 830bb05