ui: tablet shell is locked to an icon-only sidebar with no focusable expand control

[tiankaima-copilot]

Problem

Between 768 px and 1023 px, the shell shows the desktop sidebar but forces it into the collapsed icon-only state. The normal menu trigger is hidden at the md breakpoint, and the only remaining rail toggle has tabindex=-1. Keyboard and touch users therefore cannot expand the sidebar to read navigation labels.

Reproduction

1. Sign in and open any workspace page at 900×800.
2. Observe the 47 px icon-only sidebar.
3. Try to find or Tab to a sidebar expansion control.

Cause

• AppShell.svelte sets sidebarOpen=false for every viewport up to 1023 px.
• AppTopbar.svelte hides the menu trigger from 768 px upward (md:hidden).
• sidebar-rail.svelte is the remaining toggle but is removed from the tab order.

This is not the same as #532, which added aria-expanded to a trigger when one exists.

Expected

Provide a visible, focusable expansion control throughout the tablet range, or keep the mobile drawer trigger until the sidebar can remain expanded. Navigation labels must be discoverable without hover-only tooltips.

Screenshot

Local production build via Wrangler; isolated seeded database; Chrome; app commit f5d2d074407d5fc5386c165fa2b9cc849f456baf.