Feature: customizable renderers for tab overflow dropdown trigger and popover

[mathuo]

The tab overflow UI has two pieces of chrome that are currently hardcoded. Custom tab renderers already receive TabLocation = 'header' | 'headerOverflow' so the per-tab content inside the popover is customizable, but the surrounding chrome is not.

1. Overflow dropdown trigger

The chevron+count button shown in the right tab actions when tabs overflow is built by createDropdownElementHandle() in tabOverflowControl.ts and called directly at tabsContainer.ts:412. There is no option, framework binding, or public type to override it.

Proposed shape, mirroring the existing component-option pattern:

interface DockviewComponentOptions {
    createOverflowDropdownComponent?: (params: {
        tabs: number;
    }) => DropdownElement;
}

with DropdownElement (the { element, update, dispose? } shape) exported from the public API.

2. Overflow popover content

When the trigger is clicked, the popover body — scroll wrapper, group headers (color dot + label + collapsed-count badge), tab wrappers — is built imperatively in tabsContainer.ts:434-555 with no extension point. Users can re-skin individual tab entries via the existing tabComponent mechanism (because createTabRenderer('headerOverflow') is invoked per-tab), but the container and group headers cannot be replaced.

Options here, in order of increasing flexibility:

• Expose a renderer for just the group-header row.
• Expose a full popover-content renderer that receives the list of overflowed tabs/groups and returns the element, with the per-tab renderer still available as a building block.

Why bundle these

Both gaps live in the same overflow flow and would be addressed by the same kind of "renderer option on DockviewComponentOptions + public type export" pattern that already exists for watermark, tabs, etc. Doing them together keeps the API surface coherent.