feat: tab bars #105

What

A tab bar outside docking: tabs that select a page, close, reorder by drag, and scroll or shrink when they overflow. Docking (#83) reuses it for its tab stacks.

Part of #85. Needs hashed ids (#94), the state store (#95) and drag and drop (#99) for reordering.

Acceptance

• Select, close and reorder, with tests
• Overflow by scrolling arrows or a list of hidden tabs