Add an accent color picker to the editing theme

## Goal

The editing theme paints the shell in light or dark today, but buttons, focus rings, and selections are all locked to the WP admin "Modern" blue. Let users pick the accent so the workspace feels like theirs, without leaving the editing theme contract.

## Scope

* A small set of accent presets matching WP admin color schemes: Modern, Blue, Sunrise, Midnight, Coffee, Ectoplasm.
* The picker lives next to the existing light/dark toggle in the sidebar header.
* Persist the choice per user.
* Drive `--wp-admin-theme-color` and its `darker-10` / `darker-20` variants so existing components pick it up without per-component changes.
* Apply in both light and dark mode.

## Out of scope

* A free color picker. Presets only.
* Per-page or per-collection accents.
* Accent in Site Preview mode; that surface uses the active WP theme.
* Custom theming for end users on the public side.

## Acceptance criteria

* Picking an accent updates the shell immediately and survives reload.
* Buttons, focus rings, selection highlights, and links all reflect the new accent in both light and dark mode.
* The default remains the current Modern blue for users who never touch it.

Start by reading the existing editing-theme sidebar header and its light/dark toggle, then trace how theme preferences are persisted and how CSS custom properties are applied. Add the six scoped presets beside the toggle and verify that the selected accent persists after reload, updates the shell in both modes, and leaves Site Preview and untouched users unchanged.

Written by the indexing model from the issue text.