Land on your sandboxes: skip the workspace list when there is only one workspace, or remember the last one #100

What happens

At f625e302, / and every unknown path redirect to /workspaces, the workspace list. Sandboxes are one level down, on the first tab of /workspaces/:workspace. Nothing remembers which workspace you were in.

Why it matters

Most people who are not platform admins have exactly one workspace. They still see a one-row list on every visit and click through it before reaching anything they came for.

The TUI opens directly on the sandboxes of the current workspace, shows the workspace in its header, and switches with one key (w).

Proposed

• When the user can see exactly one workspace, redirect /workspaces to it.
• Otherwise, remember the last workspace opened (per browser) and land there.
• Keep the workspace list reachable from the nav, and show the current workspace in the masthead with a switcher.

Done when

• A user with one workspace lands on its Sandboxes tab after sign-in.
• A user with several lands on the one they used last, and can switch without going back through the list.