feat: add generic toolbar

[IzumiSy]

Motivation

The existing table-specific toolbar owns both DataTable controls and their layout. That prevents the same interaction model from serving other action bars and forces consumers into a fixed placement model.

This change introduces a reusable full-width Toolbar and demonstrates it in standalone and DataTable list-screen scenarios.

Design Decision

Explicit layout structure

Toolbar.Root stacks horizontal rows. Each Toolbar.Row contains related Toolbar.Group controls and optional separators; justify="between" places the row's leading and trailing groups at opposite edges without special end-slot APIs.

Seamless control participation

AppShell Button, Input, Select, Combobox, and Tabs controls register their focus target through the row context. Plain controls participate in row-level Arrow/Home/End navigation, while composite controls keep their own directional-key behavior.

DataTable composition

DataTable.ColumnSettings is now placeable. A generic Toolbar directly inside DataTable.Root becomes the table's top bar: its side/top borders are removed while its top corners and bottom divider are retained.

Summary

• Add the public Toolbar component, its documentation, tests, and a runnable example page.
• Make existing AppShell controls toolbar-aware without consumer wrappers.
• Compose the generic toolbar into DataTable examples and expose DataTable.ColumnSettings.