MarkdownEditor toolbar: use Radix Toolbar from radix-ui instead of the hand-rolled roving tabindex

## Context

PR #157 gave `MarkdownEditor`'s formatting buttons `role="toolbar"`, real names, and a hand-rolled roving tabindex (`activeButton` state + a keydown handler for Arrow/Home/End).

## Problem

The roving tabindex is ~30 lines of bespoke focus management that duplicates what Radix's `Toolbar` primitive already does (roving focus, orientation, `Home`/`End`, RTL, and correct `tabIndex` bookkeeping). `radix-ui` is already a dependency of `apps/web` (`radix-ui@^1.4.3`) and exports `Toolbar`, so no new package is needed.

## Proposal

Replace the `

Start with the MarkdownEditor implementation and compare its toolbar markup, activeButton state, toolbarRefs, and keydown handler with the Radix Toolbar API. Run apps/web/tests/MarkdownEditor.test.tsx; done means the hand-rolled focus management is replaced while the existing single-tab-stop and Arrow/Home/End behavior still passes.

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