Studio settings: replace the phone section select with the prototype's section index #2261

Parent: #2210. Related: #2201 (no native controls). Found in #2252 and #2256.

Problem

On phones (below 500px), the Settings section picker is still a native <select>, restyled. Two reasons it stayed:

• settings-accessibility.test.tsx reads its static <option> list and fires a native change event. A Radix Select renders no options until opened, so migrating it isn't a selector-only test change.
• /workspace/settings redirects to profile.

The prototype doesn't use a select here at all. On phones it shows a section index (the list of sections, grouped with icons), and each section page has a back bar.

Expected

Phone Settings navigation matches the prototype: an index page of sections, and a back bar on each section. No native <select>.

Scope

• Phone-only index at /workspace/settings. Desktop keeps redirecting to profile, or follows the prototype if it differs there. Plus a back bar on each section page.
• Keep the slugs and legacy redirects.
• Update settings-accessibility.test.tsx so it asserts the same reachability (every section reachable, current section announced) without depending on a native select.
• Screenshots: Before | After | Prototype at 390px, light and dark.